Heirloom Tomato and Caramelized Onion Tart

Do you remember “gas wars” where competing neighborhood filling stations lowered their prices by a penny at at time to competitively beat their neighbor? My favorite neighborhood grocery ran “loss leader” specials over the Fourth of July week. I’m not sure if was due to the opening of a competing neighborhood grocery store or just because of the holiday. Anyway, I was able to purchase beautiful heirloom tomatoes for $.99 per pound. Now, that’s a bargain.

A ripe tomato is my all-time favorite summer-time vegetable. I came home with a couple of pounds and made a fantastic tomato and caramelized onion tart — one of my favorite summer recipes yet.

What are heirloom tomatoes?

“Heirloom tomatoes” have become popular in recent years. They are characterized by various colors — purple, red, orange, greenish — and unusual shapes. I like them because of their sweetness, juiciness and true tomato flavor.  Heirloom tomato varieties are “open pollinated” and cross-pollination (done by wind or bees) occurs without human intervention. The seeds are handed down from generation to generation. With hybrid tomatoes, growers cross-pollinate two different varieties of tomatoes to intentionally produce desired characteristics such as higher yield, thicker skin (easier for transport), improved disease resistance, etc.

Heirloom tomatoes varieties are prone to be less disease resistant and they can split. Hence, they are more difficult to grow and transport and are more expense at the market. It is not unusual to find the selling price at $5.99 a pound. I purchased these for $0.99 a pound.

These heirloom tomatoes were grown in a hot house. Hum, I wonder how much open pollination (from wind or bees) occurred with these tomatoes. This goes to show that the use of the term “heirloom” is taken loosely by growers these days. Buyers beware.

Heirloom Tomato and Caramelized Onion Tart

I decided to use some of my heirloom tomatoes to make a tart. The combination of tomatoes and onions sounded really tasty. After searching the internet for various ideas, I came up with my own “hybrid” recipe. This turned out to be about the best recipe I’ve made so far this summer. As I said, I love tomatoes. Adding caramelized onions, cheese and herbs with the tomatoes on a simple crust — the tart quickly vanished.

Making the Crust for the Tart

Several different types of crusts can be used to make this tart. Some recipes called for puff pastry dough and others used pie-type pastry crusts — often purchased convenience frozen pie crusts. I guess I was too lazy to drive to the store to get a convenience pie crust shell. A homemade pie crust is really easy to make using a food processor. I have the technique perfected to where it works every time. (See the technique on my blog post, “Classic Deep Dish Apple Pie.”) The crust turns out to be flaky and savory — it set off the filling really well.

So I made my own pie crust. Here I go. I make it in a food processor. You need flour, a pinch of salt, a stick of solid margarine and ice water. Blend the flour and salt together in the food processor bowl, cut in the margarine with “pulses” and lastly add in ice water — a tablespoon at a time. The trick — use ice water.  And don’t use more ice water than I have shown in the recipe, just keep the food processor running and swirling until the dough begins to pull away from the edges of the food processor and form into a ball. Then roll out on a floured board (I use a large cutting board). Keep some flour on the cutting board below the pie crust. Lift up the crust if you need to and sprinkle on more flour. This tart is larger than a typical pie shell and so I adjusted the flour and ice water in the recipe are proportionally. The crust needs to be rolled to approximately 10″ x 10″.

Loosely drape the crust over the rolling pin, transfer to a baking sheet which is covered in aluminum foil and unroll. Who has a square baking sheet? I don’t and so I used a rectangular one.

Use your fingers to shape the crust into a 10″ x 10″ square and form the fourth edge. Use a knife to trim off the excess dough and use these scraps to fill in thin areas.


First make caramelized onions. Slice them thinly, cook them in olive oil in a covered skillet on medium heat for about 15 to 20 minutes. Check them and stir frequently. Once the onions start to brown, they quickly burn. I added thyme to the onions for extra flavor.

Add arrange the caramelized onions on the tart dough.

Next onto the tart is cheese. I found that a variety of exotic cheeses are used in tomato and cheese tarts — but then that’s another trip to the grocery store. I used low-fat mozzarella cheese that I had on hand. Since I wanted the flavors to be about the tomatoes and onions, I only added a little cheese.

Lastly, top with thin (about 1/4″) heirloom tomato slices. These were huge tomatoes; I didn’t need many slices and used the remaining slices of tomatoes just “for eating.”

Sprinkle on dried basil and bake in a 375 degree onion until the crust is browned which is about 35 to 40 minutes. Remove from oven, carefully lift up the aluminum foil (yes, it does serve a purpose) and place onto a very large cutting board. Then carefully slide the tart off the foil and onto a serving platter. Yum. I’m bragging about this recipe. I addition to tasting great, it is really my own concoction.

Do you have to use heirloom tomatoes? No, any type of ripe tomatoes will work. If the tomatoes are excessively juicy, I suggest first draining them on paper towels before adding to the tart to remove the juices. Here’s a tart I made using vine ripe tomatoes.

Enjoy the tart! Summer is here in Louisiana. Hot weather. Be careful outside. Have lots of water to drink for yourself as well as children, the elderly and pet dogs. Anyway, I love summer — its my favorite time of the year.

Heirloom Tomato and Caramelized Onion Tart

  • Servings: 9 servings
  • Difficulty: moderate
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    • 1 recipe of “Pie Crust Dough” ( see recipe below) or two purchased, frozen pie crust shells or sheets
    • 2 Tbsp olive oil
    • 4 cups thinly sliced (2 large or about 1 lb) white or yellow onions
    • 1/2 tsp dried thyme
    • 1/2 tsp salt
    • 1/2 cup shredded low fat Mozzarella cheese
    • approx 1-1/2 lb heirloom assorted tomatoes
    • 1 tsp dried basil

Method and Steps:

  1. Pre-heat oven to 375 degrees.
  2. On floured pastry or cutting board, roll pie crust dough out into an approximate oval or square as least 10″ x 10″ in size. Or roll purchased and defrosted pic crust shells out into 10″ square shape.
  3. Line a baking sheet with aluminum foil. Carefully drape the pie crust around the rolling pin and transfer to prepared baking sheet. Use a knife to trim the edges.Use your fingers to create fourth edge of tart and use scraps to fill in thin areas of crust. Set aside.
  4. Add olive oil to large skillet and add thinly sliced onions. Turn heat to medium high and stir to coat onions with oil.
  5. Reduce heat to medium and cover skillet. Cook until onions are tender and translucent about 15 to 20 minutes. Check and stir often. Add dried thyme and salt.
  6. Remove lid, continue to cook and stir until onions begin to turn golden brown, about 5 additional minutes. Check often and  immediately remove from stove.
  7. Arrange caramelized onions on top of tart pastry.
  8. Sprinkle shredded Mozzarella cheese over onions.
  9. Peel skins off bottoms and tops of heirloom tomatoes (but not sides of tomatoes) and cut into 1/4″ slices. Cut enough tomatoes to fill the tart and arrange the tomatoes in overlapping rows. Save tops and unused tomatoes for another use.
  10. Sprinkle with dried basil.
  11. Bake tart in 375 degree oven until the tart shell is nicely browned, about 35 to 40 minutes.
  12. Remove from oven, gently lift up the aluminum foil and transfer to a very large cutting board. Then gently slide tart onto a serving platter. Cut into small squares, either three or four crosswise and lengthwise, and serve.

Homemade Pie Crust

  • Servings: one 10
  • Difficulty: moderate
  • Print


    • 2 cups all purpose flour
    • 1/4 tsp salt
    • 1/2 cup (1 stick) solid margarine
    • 1/4 cup plus 1 tbsp ice water

Method and Steps:

  1. Add flour and salt to medium food processor bowl and mix with mixing blade.
  2. Add shortening, cut in chunks, through shoot.
  3. Turn on pulse cycle and pulse 4 cycles. Shortening will be cut into small particles about the size of small peas.
  4. With food processor on continuous cycle, add ice water through shoot and mix  until pastry is moistened. Don’t add extra water. Then continue to process until the dough just begins to pull away from the sides to form into a ball.
  5. Remove pastry and place on a floured board. Using hands, form into a ball. Roll out immediately or chill/freeze  for future use.




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