In a medium skillet, melt 1 tablespoon (14 grams) butter over medium heat. Add leeks and garlic. Cook, stirring frequently, until leeks have softened and mixture is lightly browned and fragrant, about 5 minutes. Remove from heat, and let cool completely. (You should have a scant ½ cup mixture after cooking.)

Line a rimmed baking sheet with parchment paper.

In a small bowl, stir goat cheese until smooth and well combined.

In a large bowl, toss together squash, oil, salt, and pepper until well combined.

Turn out Flaky Pie Dough onto a lightly floured surface. Roll into a 13-inch square (about ¼ inch thick). Using a sharp paring knife, cut 4 (6-inch) rounds from dough. Place rounds onto prepared pan. Score a 4-inch circle in center of each dough piece, making sure not to cut all the way through. Using a small offset spatula, spread about 1½ tablespoons (28 grams) goat cheese onto each dough piece within 4-inch circle. Top each with 1½ tablespoons (20 grams) leek mixture. Top each with about ¾ cup (105 grams) squash, overlapping slices as desired. Freeze until firm, at least 15 minutes.

Position oven rack in bottom third of oven. Preheat oven to 425°F (220°C).

In another small bowl, whisk together egg and 1 tablespoon (15 grams) water; brush onto borders of each dough piece. Cube remaining 1 tablespoon (14 grams) butter; top each galette evenly with butter pieces.

Bake for 10 minutes. Reduce oven temperature to 400°F (200°C), and bake until crust is golden brown, 15 to 20 minutes more. Let cool on pan for 15 minutes. Garnish with thyme, if desired. Best served warm. Refrigerate in an airtight container for up to 2 days.