Stir the flour, sugar, and salt together in a large bowl. In a measuring cup, add the apple cider vinegar to the ice water.

Add the butter to the flour mixture and toss until each piece is coated. Use your fingertips to press each cube of butter into a flat sheet. Keep tossing to coat.

Sprinkle about 4 tablespoons of the vinegar water over the flour mixture. Use a fork or your hands to gently stir the water into the flour mixture to make a shaggy dough.

Press the dough together, form it into a disk, and wrap it tightly in plastic wrap. Chill the dough for at least 2 hours.

Heat oven to 375 degrees and line a baking sheet with parchment paper.

In a small bowl, combine the goat cheese, chives, 1 teaspoon thyme leaves, and a sprinkle of salt and pepper. Stir to blend well.

In another bowl, combine the butternut squash, onion, olive oil, paprika, remaining 1 teaspoon thyme, and a sprinkle of salt and pepper; toss to coat.

On a lightly floured surface, roll the dough into a 12-inch round just under ?-inch thick and place it on the baking sheet. Dollop the goat cheese mixture over the top and spread it into a circle, leaving a 2-inch border.

Arrange the butternut squash and onion in an even layer on top of the goat cheese. Fold the crust up and over the squash and press gently to seal. Brush the crust with egg and sprinkle with salt and pepper.

Bake the galette until the crust is golden and the squash is cooked through and charring on the edges, 35 to 45 minutes. If necessary, tent the galette with foil.

Let the galette rest on the baking sheet for about 5 minutes before moving. Drizzle the filling with olive oil, if desired.