Caramelized Red Onion Tart with Seasonal Vegetables

Tarts are some of my favorite things to bake…and eat. With the perfect balance of crust and filling, this red onion version makes use of a variety of vegetables and cheeses already on hand – a handful of green beans, a few cherry tomatoes just about to become too ripe. They combine beautifully to top a caramelized onion layer. Tucked beneath the vegetables are goat cheese and feta as well as Parmesan, which lends a nutty quality.

I love dishes such as this one that change throughout the seasons.  Certainly the assorted leftover vegetables in my kitchen could make a salad, but here the beautiful slices are celebrated even more so.

Ingredients

Pastry

1 cup all-purpose flour
¼ cup whole wheat or graham flour
½ teaspoon kosher salt
6 tablespoons unsalted butter
1 egg yolk
3 tablespoons ice water (plus more as needed)

Filling

3 red onions
3 tablespoons olive oil
½ teaspoon dried rosemary leaves
Kosher salt to taste
2 tablespoons red wine
1 tablespoon brown sugar
Assorted vegetables (carrot, squash, cherry tomatoes, green beans, or any combination you have on hand)
3 sprigs fresh thyme
3 tablespoons grated Parmesan cheese
Goat cheese and feta to preference

A Note About the Crust Flours: For this recipe, I incorporate whole wheat flour or graham flour, a coarsely ground whole grain flour with nutritional and flavor value. Tart dough made with graham or whole wheat flour may require more liquid. It will continue to hydrate during its initial rest in the refrigerator, but if you notice any dry spots while forming the dough initially, simply spritz them with a bit of water.

How to Make the Tart Pastry

Combine the flours and salt in a food processor.  Cut the butter into small pieces and pulse into the flour mixture until it resembles coarse crumbs.  Whisk egg yolk and water. Add to flour mixture and pulse to combine. [If dough doesn’t hold together when pressed, add small amounts of water slowly.]  Turn onto work surface or plastic wrap and gently work into a cohesive disk. Refrigerate wrapped dough for at least 30 minutes. In the meantime proceed to the caramelized red onion layer.

How to Caramelize the Onions

When caramelizing onions, the key is to cook them moderately low and slow, as their natural sugars take a while to brown and develop a richer sweetness.  First I start them over medium-high heat just to begin the softening process, being careful not to brown them.  Heat the oil in a large skillet over medium-low heat, and add about half the onions.  Once they just start to soften, add the rest, and sprinkle with kosher salt.  

Reduce heat to medium-low and cook, stirring every few minutes, for about 45 minutes to an hour.  The onions need to remain in contact with the hot surface to caramelize, but if you find that they are cooking too quickly, adjust the heat accordingly.  As they cook, you can expect to see some browned bits on the bottom of the pan.  Add water, a tablespoon at a time, and scrape these from the bottom of the pan with a wooden spoon as needed throughout the process.

When the onions are very soft but not mushy, add the rosemary and a pinch of salt to taste and cook for another minute.  Next add the red wine to deglaze the pan once again, followed by the brown sugar. Allow to cool.

How to Assemble the Vegetable Tart

Once the dough has chilled, allow it to rest briefly so that it does not crack when rolled. Roll on a lightly floured surface then line a 9-inch tart pan with removable bottom.  Fold any excess dough in and crimp to reinforce top edge. Dock the bottom of the dough with a fork. Cover and refrigerate for 30 minutes. While the dough chills, preheat the oven to 350 degrees.

Line the tart with chilled tart with parchment paper and fill with beans or pie weights. Bake for 12 minutes.  Remove the foil and weights, and bake for 7 more minutes. Remove the crust from the oven, let cool for at least 15 minutes, and raise the temperature to 375 degrees.

In the meantime, prepare assorted vegetables you have on hand.  Squash and other vegetables with a high water content can be sliced thinly and set aside.  Cherry tomatoes can be halved. Harder vegetables such as carrots should be sliced thinly then blanched by cooking for 1 to 2 minutes in a small pot of boiling water then rinsed under cold water. Green beans should be sliced very thinly on a diagonal. 

Add 2 to 3 tablespoons of olive oil based on how much it takes to coat the amount of sliced vegetables you have. Season with thyme leaves and a sprinkle of salt.

Sprinkle Parmesan over cooled crust.  Top with caramelized red onions and use a fork to spread evenly.  Dot with goat cheese, feta, or other varieties that don’t become stringy when heated.  Arrange assorted vegetables as top layer.

Bake for 30 minutes and serve warm.

Reheating tip:  Preheat oven to 325 degrees. Cover tart loosely in foil and heat until warm.