Preheat the oven to 425°F (220°C). Peel and cube the butternut squash, toss with olive oil, salt, and cayenne pepper, and roast for 15-20 minutes until tender.

In a skillet, heat olive oil and sauté the sliced red onions for 3-5 minutes until translucent.

Reduce oven temperature to 350°F (175°C). Roll out the galette dough, spread Boursin cheese on it, and top with roasted squash, onions, and fresh sage leaves.

Fold the edges of the dough over the filling, brush with heavy cream, and sprinkle with grated Parmesan and black pepper.

Bake for 45-50 minutes until the crust is golden brown. Let cool for 5-10 minutes before serving warm.