1 medium tomato (9 ounces; 255 g), cored and sliced 1/4 inch thick
Kosher salt
Herbes de Provence, for sprinkling (optional)
Extra-virgin olive oil, for drizzling
Preheat oven to 400°F (200°C) and set rack in middle position. If Using Puff Pastry: On a lightly floured surface, gently roll out the puff pastry until it’s large enough to line an 8- or 9-inch tart pan or a standard 9-inch pie plate, roughly the size of an 10-inch round. Carefully transfer the mixture to a tart pan or pie plate, gently pushing it into the edges, and trim any excess overhang. Dock all over with a fork, then proceed to Step 2. Pie Dough: for single-crusted pies; Note, you can also use this style of dough for a free-form galette
Spread mustard all over base of tart, then sprinkle Gruyere all over.
Arrange tomatoes on top of mustard and cheese, then season all over with salt. Sprinkle lightly with herbes de Provence (if using) and drizzle with olive oil.
Bake until tomatoes are roasted and excess water has cooked off and pastry is browned and crisp all over, about 30 minutes. Let cool slightly, then slice and serve.
The tart is delicious with plain Dijon. If you want, you can cut it with an equal-parts mixture of crème fraîche, which is also delicious, though by no means necessary.
