- 1 piece frozen puff pastry, thawed
- 1 egg white
- 4 medium tomatoes (heirloom in different colors)
- 3 tablespoons whole grain mustard
- Sea salt
- 1 shallot, thinly sliced and slices separated into rings
- 1 teaspoon chopped fresh thyme
- Preheat the oven to 375°F.
- Lightly grease or line a baking sheet with parchment paper or a silicone liner.
- On a lightly floured surface, roll the dough out to about a 12- by 15 rectangle. Place on the prepared pan and flip over a 1/2-inch border, using water to hold down the edges and then crimping with a fork.
- Whisk the egg white with 1 tablespoon water, and brush this over the edge of the tart. Prick the inside of the tart in several places with the fork. Par-bake until lightly browned and slightly cooked in the center, 12-15 minutes. Let cool slightly.
- Meanwhile, cut the tomatoes in half through the equator, then gently squeeze to remove seeds and excess liquid. Slice into 1/8 to 1/4 inch-thick slices.
- Spread the prepared crust evenly with the mustard, then layer the tomatoes on top in alternate colors, overlapping them by about 1/2 inch and arranging them tightly within the edges (they will shrink a bit). Season well with sea salt, then top with the shallot rings and thyme.
- Cook until the tomatoes are softened and the crust is cooked through in the center, 20-25 minutes.