Winter Squash Galette with Caramelized Onions and Gruyère

Ingredients

• Galette Crust
• 2 cups all-purpose flour, plus more for dusting
• 1 teaspoon kosher salt
• 1½ sticks cold unsalted butter, cubed
• 1 tablespoon apple cider vinegar
• ½ cup ice water
• Filling
• 1½ pounds winter squash (such as butternut or kabocha)—peeled, seeded and sliced into ¼-inch wedges
• Extra-virgin olive oil, as needed
• Kosher salt and freshly ground black pepper
• 1 medium yellow onion, thinly sliced
• 4 ounces Gruyère cheese, grated
• 1 garlic clove, thinly sliced
• 1 large egg, lighten beaten
• 2 teaspoons fresh thyme leaves
• Flaky salt, to garnish