Thaw one package frozen puff pastry according to package directions, preferably in the refrigerator. Thaw 10 ounces frozen spinach.

Arrange a rack in the middle of the oven and heat the oven to 375°F. Line a rimmed baking sheet with parchment paper.

Prepare the following, adding each to the same medium bowl as you complete it: Crumble 8 ounces feta cheese (about 1 ¼ cup), then mash with a fork until a coarse paste forms. Squeeze the excess water from the spinach. Finely grate 2 garlic cloves.

Add 1 large egg, a pinch of kosher salt, and ¼ teaspoon black pepper. Mix until evenly combined.

Unfold the puff pastry on a work surface. If using a puff pastry that comes in one sheet, roll it out into a 20-inch square, dusting the work surface with flour if needed. (If there are two sheets, no need to roll out.) Cut into 8 (about 5-inch) squares. Divide the filling evenly between the pastry squares (about ¼ cup each). Fold each pastry square in half over the filling to form a triangle and lightly press the edges together (they won’t stick together, which helps create flaky edges). Place on the baking sheet, spacing them evenly apart.

Place 1 large egg yolk and 2 teaspoons water in a small bowl and whisk together with a fork. Brush the egg wash over the tops of the bourekas. Sprinkle with 1 teaspoon white sesame seeds and 1 teaspoon nigella or black sesame seeds if desired.

Bake until the bourekas are puffed and golden brown, 20 to 25 minutes. Let cool for at least 5 minutes on the baking sheet before serving.