Preheat an oven to 375 degrees F. Defrost 1 sheet frozen puff pastry (preferably all-butter pastry) from a 14–17-oz. box according to package directions. If the package contains 1 sheet, cut the pastry in half; if the package contains 2 sheets, just use 1. Roll out the dough slightly on a floured surface to smooth it out.

Place the pastry sheet on a parchment-lined, rimmed baking sheet and use a paring knife to gently score a 1/4” border around the edge. Using a fork, prick the pastry all over inside the border to release steam while baking.

Spread the bottom of the dough with a thin layer of Dijon mustard, and then cover with a single layer of snugly packed tomato slices, preferably heirloom tomatoes in several colors, that have been sprinkled with sea salt and drained on a paper towel to remove excess moisture. Drizzle with extra-virgin olive oil and sprinkle with freshly ground black pepper and chopped fresh thyme. Top with grated Gruyere cheese. Bake until the crust is golden and the tomatoes have collapsed. It should take about 20 to 25 minutes. Check at 20 minutes and continue to cook until pastry is golden. Remove the tart to a rack and let it cool for 20 minutes or so before slicing it into wedges.

Variations: Add slivers of kalamata olives over the cheese before baking. Or sprinkle the cooked and cooled tart with torn bits of fresh basil right before serving.