Preheat your oven to 425°F (220°C). In a large bowl, combine all-purpose flour and salt. Using a pastry cutter or your fingers, cut in cold butter until it resembles coarse crumbs. Gradually add ice water, mixing until the dough just comes together. Form into a disc, wrap in plastic wrap, and chill in the refrigerator.

On a parchment-lined baking sheet, toss peeled and diced butternut squash with olive oil, garlic powder, onion powder, and a pinch of cayenne pepper. Spread the squash in a single layer and roast for about 15-20 minutes, or until tender and lightly caramelized.

In a skillet over medium heat, add a splash of olive oil and sauté finely chopped red onion for about 3-5 minutes until translucent. Season with a pinch of salt. Remove from heat and set aside to cool briefly.

Lower the oven temperature to 350°F (175°C). On a floured surface, roll out the chilled dough into a 12-inch circle. Transfer the dough to a parchment-lined baking sheet. Spread Boursin cheese over the base, leaving a border. Layer on the roasted butternut squash and sautéed onions. Sprinkle fresh sage leaves over the filling and fold the edges of the dough over the filling.

Brush the crust with heavy cream and sprinkle grated Parmesan cheese evenly over the galette. Bake for 45-50 minutes, until the crust is golden brown and flaky, and the filling is bubbling. Allow to cool for about 10 minutes before slicing.