Instructions

1. Preheat the oven to 375F. Line a 9-x-13 baking sheet or baking pan with parchment paper.
2. Par-bake the puff pastry: Lay the thawed puff pastry on the baking sheet, and trim off any overhang. Prick the pastry with a fork (to prevent it from puffing up like crazy), then line with a second layer of parchment. Fill the baking sheet with pie weights, dried beans or uncooked rice. Bake for 20-25 minutes, until the edges of the pastry are puffed and golden brown. Carefully remove the weights and the parchment.
3. While the pastry is par-baking, caramelize the onions: in a large skillet, melt 3 tablespoons of the butter on low heat. Add the onions and sugar and caramelize, stirring frequently, until the onions are a deep golden brown. (You can walk away every now and then early on, but as the onions really start to take on color, you’ll want to watch them like a hawk. It’s a fine line between caramelized onions and burnt onions.) Transfer the caramelized onions to a plate.
4. In the skillet, melt the remaining tablespoon of butter. Increase the heat to high, then add the tomatoes, the remaining teaspoon of salt and the leaves from 2-3 sprigs of thyme. Roll the tomatoes around a few times until they start to slump and burst and pick up some color.
5. After a few minutes, turn off the heat and let the tomatoes cool slightly.
6. Assemble the tart: sprinkle the grated Gruyere onto the par-baked puff pastry, then scatter the caramelized onions over top. Toss the tomatoes on top, then scatter the remaining leaves of thyme.
7. Increase the oven heat to 425F. Bake the tart for 15-20 minutes, until the edges are browned and the tomatoes have charred slightly. Let cool for 10 minutes, then cut into squares and serve.