Preheat your oven to 425°F (220°C). Cut the squash into uniform cubes and arrange them on a lined baking sheet. Drizzle with olive oil, salt, pepper, and cayenne pepper. Roast for about 15-20 minutes until tender.

In a medium skillet, heat a splash of olive oil over medium heat. Add thinly sliced red onion and sauté for 3-5 minutes until translucent and fragrant. Set aside to cool.

Lower the oven temperature to 350°F (175°C). On a lightly floured surface, roll out the dough into a ¼-inch thick circle. Transfer to a parchment-lined baking sheet.

Spread a layer of Boursin cheese on the dough, leaving about a 1-inch border. Layer the roasted butternut squash and sautéed onions evenly over the cheese. Sprinkle with sage leaves.

Fold the edges of the dough over the filling to create a rustic crust. Brush with heavy cream and sprinkle with Parmesan cheese.

Bake in the preheated oven for 45-50 minutes until the crust is golden brown and crisp. Allow to cool for 5-10 minutes before slicing.

Slice the galette into wedges and serve warm, optionally topped with crumbled gorgonzola.