Preheat the oven to 425°F (220°C) with a rack positioned in the center. Line a sheet pan with parchment paper.

In a large bowl, stir together the brown sugar, coconut oil, vinegar, and kosher salt.

Slice the squash into 1-inch (2.5 cm) wedges or ½-inch (13 mm) chunks and toss with the sugar mixture.

Roll the dough into a round or rectangle and set it on the lined sheet pan using the Basic Method.

Sprinkle about half of the crumble topping over the crust in an even layer, leaving a 2-inch (5 cm) border.

Leaving any excess liquid in the bowl, arrange the squash over the crumble in a single layer.

Sprinkle over the remaining crumble topping in bite-size chunks.

Fold the edges of the crust over the filling toward the center, overlapping and pleating as desired.

Freeze the galette on the sheet pan for 10 minutes.

Remove the galette from the freezer and brush the egg wash over the exposed crust.

Sprinkle the crust with demerara sugar.

Bake until the crust is starting to turn golden, 12 to 15 minutes.

Reduce the oven temperature to 375°F (190°C) and continue to bake until the squash is tender and the crust is deeply golden brown, another 45 to 55 minutes, rotating the sheet pan front to back halfway through.

Remove the galette from the oven and sprinkle with some flaky sea salt.

Cool, uncovered, for 30 minutes or up to 8 hours. Slice and serve.

Leftovers can be stored at room temperature, loosely covered, for up to 2 days. Reheat on a sheet pan in a 350°F (180°C) oven until warmed through, about 10 minutes.