Savory Caramelized Onion and Two-Tomato Tart

1 tablespoon unsalted butter 1 medium onion, halved and thinly sliced 2 garlic cloves, smashed 2 thyme sprigs, stems removed and discarded ¾ teaspoon kosher salt, divided 1 cup mixed red and yellow grape tomatoes, halved 1 sheet prepared puff pastry, defrosted egg wash (1 large egg and 1 teaspoon water, whisked together) ½ cup plus 1 tablespoon shredded Comte cheese, divided ¼ cup fresh ricotta, strained of any excess water 2 large basil leaves, thinly sliced or torn into small pieces

1.  Put the butter, onion, garlic, thyme and ½ teaspoon of the salt into a medium skillet set over low heat.  Cook until the onions are deeply golden, stirring occasionally – the onion must cook slowly, so if it seems as though it is browning too quickly or getting too dark, lower the heat.  This process will take about 45 minutes to 1 hour.

2.  While the onion is caramelizing, toss the tomatoes in a small bowl with the remaining salt; reserve.

3.  Line the baking pan with parchment paper.  Roll out the sheet of pastry on a well-floured surface; cut to fit pan and place on the prepared pan, folding over the edges to make a border.  Using the tines of a fork, prick the surface of the tart shell all over, in the center and on the border.  Brush the egg wash all over the tart shell.

4. Preheat oven on Convection Bake at 400°F with the rack in the lower position.  Place the empty tart shell on the rack and bake for 20 minutes. Keep an eye on the shell to make sure the surface does not get too dark. Cover with aluminum foil, if it gets dark before time expires. 

5. Once shell is baked and cools slightly, press down center of tart shell slightly to keep edges up.  Scatter ¼ cup of the shredded cheese along the bottom of the tart shell.  Put ½-tablespoon dollops of the ricotta on top of the cheese evenly across the tart; sprinkle with half of the remaining salt. Top evenly with the onion mixture and then the tomatoes, and finally the remaining cheese and salt.  Place back into the oven in the upper rack position. Set oven to Broil for about 8 to 10 minutes, until cheese is melted and golden brown.

6. Scatter the basil on top of the tart before serving.