01 - In a large bowl, combine flour and salt. Cut in cold butter using a pastry blender or fingertips until mixture resembles coarse crumbs. Gradually add ice water, one tablespoon at a time, mixing until dough holds together. Shape into a disc, wrap in plastic wrap, and refrigerate for at least 30 minutes.
02 - Heat oven to 400°F. Line a baking sheet with parchment paper.
03 - In a bowl, toss butternut squash and sliced onion with olive oil, thyme, nutmeg, salt, and pepper. Spread in a single layer on the prepared baking sheet and roast for 20 minutes.
04 - Remove baking sheet from oven, add apple slices, toss gently, and roast for another 15 minutes until squash is tender and apples slightly softened. Allow to cool briefly.
05 - Lightly flour a surface and roll out chilled dough into a 12-inch circle. Transfer to the parchment-lined baking sheet.
06 - Spread roasted squash, onion, and apple mixture evenly over dough, leaving a 2-inch border. Sprinkle goat cheese and parsley over filling.
07 - Fold dough edges up and over filling, pleating as necessary. Brush crust with beaten egg and sprinkle with coarse sea salt if desired.
08 - Bake galette 30 to 35 minutes until crust is golden brown. Let cool slightly before slicing and serving.