Spinach-and-Cheddar Slab Quiche
Unbleached all-purpose flour, for dusting
1 sheet (14 ounces) frozen all-butter puff pastry, such as Dufour, thawed
10 large eggs, plus 1 egg white
1/4 cup white sesame seeds
10 ounces frozen chopped spinach, thawed, drained, and squeezed dry
3 ounces cheddar, coarsely grated (1 cup)
1 1/4 cups heavy cream
Kosher salt and freshly ground pepper