1 savory pastry crust
3 Tablespoons olive oil
2 large yellow onions
1 teaspoon salt
2 teaspoons minced garlic
2 large ripe tomatoes, like Beefsteak or Big Boy (about 1 pound)
1 cup grated Gruyere cheese (2.5 ounces)
3 Tablespoons pesto
Freshly ground pepper
1/4 cup grated Parmigiano Reggiano (.5 ounce)
1 large egg yolk
1 tablespoon milk
2 Tablespoons extra virgin olive oil, garnish
Fresh basil leaves, cut into thin ribbons, garnish
Roll the pastry dough into an 11-inch circle that is 1/8-inch thick. Line a cookie sheet with a sheet of parchment or wax paper. Transfer the pastry circle to the sheet pan and refrigerate at least 2 hours. (The best way to transfer the dough is to fold it in half, then fold in half again, so the dough is easier to pick up. Open the folded pastry into a flat circle on the parchment-lined sheet.)
Cut the onions in half through the stem end, then slice them crosswise into very thin slices (about 5 cups). Heat the oil in a large non-stick skillet over medium-high heat. Add the onions and stir continuously for 2 minutes. Reduce the heat to medium-low and add the 1 teaspoon of salt. Cook the onions, stirring occasionally, until they are golden and very soft, 20 to 30 minutes.
Add the garlic and cook another 2 minutes, stirring frequently. Remove the skillet from the heat. Cool to room temperature or chill, covered, for several hours or overnight.
Remove the stem ends from the tomatoes and cut them crosswise into 1/8-inch slices. Place the slices on several thicknesses of paper towels to rid of excess juices.
Position a rack in the bottom of the oven and preheat to 400 degrees F.
Remove the cookie sheet with the pastry circle from the refrigerator. Spread the Gruyere across the pastry, leaving a 1-inch border uncovered. Scatter the onion mixture on top of the cheese. Carefully spread the pesto over the onions, then top with a single layer of slightly overlapping tomato slices (you may not use all of the tomato slices). Sprinkle the tomatoes with salt and pepper.
Place the egg yolk and milk in a small cup and whisk to blend.
Fold the edges of the pastry over the filling, pinching and pleating the dough to form a rim that encases the filling. Brush the edges of the pastry with the egg wash. Sprinkle the Parmigiano Reggiano over the tomatoes and pastry edges.
Bake until the juices bubble and the crust turns golden brown and crispy, 30 to 40 minutes.
Remove the cookie sheet from the oven and slide the tart (with the paper) onto a wire rack. Let cool for 10 minutes before slicing into wedges. Garnish with a drizzle of olive oil and a sprinkling of basil, if desired.
Pairs Well With
Served warm from the oven, this is yummy tart makes a terrific first course, or pair with a simple salad for a tasty lunch or supper. Summer-ripe tomatoes(the ones in the photo are my home grown maters) are essential for the best flavor, although somewhat lackluster tomatoes will get a boost from the hidden filling of caramelized onions, pesto, and nutty Gruyere cheese. The crust gets soggy quickly, so this is not a recipe that can be made far in advance of serving. If you have leftovers, warm the tart in a hot oven (425 degrees F) on a baking sheet that has been preheated.