01 - Whisk together flour and salt in a large bowl. Cut in cold butter using a pastry blender or your fingers until mixture resembles coarse crumbs. Gradually add ice water, mixing just until dough comes together. Form into a disk, wrap in plastic, and refrigerate for 30 minutes.
02 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
03 - Toss butternut squash, apples, and red onion with olive oil, thyme, salt, and pepper. Spread on prepared baking sheet and roast for 20-25 minutes, stirring once, until squash is tender and golden. Cool slightly.
04 - On a lightly floured surface, roll out chilled pastry to a 12-inch circle. Transfer to a parchment-lined baking sheet.
05 - Arrange roasted squash, apples, and onions in center of pastry, leaving a 2-inch border. Sprinkle with goat cheese and drizzle with honey if desired.
06 - Fold pastry edges over filling, pleating as needed. Brush crust with beaten egg and sprinkle with coarse sea salt if desired.
07 - Bake for 25-30 minutes until crust is golden brown. Let cool for 10 minutes before slicing and serving.