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.