In a pot over medium heat, cook the onion and garlic in the oil until translucent. Add 2 cups (280 g) of the squash, the broth and cayenne pepper. Bring to a boil. Cover and simmer for 20 minutes. Transfer the mixture to a blender, add the Parmesan and purée until smooth. Lightly season with salt and pepper. Set the sauce aside.
Meanwhile, with the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with a silicone mat or parchment paper.
Place the prosciutto on the baking sheet without overlapping. Top each slice with a sage leaf. Bake for 7 minutes or until lightly golden. Let cool on a wire rack.
In a pot of salted boiling water, cook the pasta for 4 minutes. Add the remaining squash and cook for 5 minutes or until the pasta and squash are al dente. Drain. Return the pasta and squash to the pot. Add the sauce and mix well.
Transfer to an 11 x 8-inch (28 x 20 cm) baking dish with an 8-cup (2 litre) capacity. Add the mozzarella and press into the mixture. You can cover and chill at this stage, if desired.
Bake for 10 minutes (or 25 minutes if the dish was refrigerated) or until the mozzarella is melted. Cover with the prosciutto slices and serve.
Cooking the chunks of squash in the same pot as the fusilli means less work, less washing, less worry. Make and refrigerate the sauce up to 72 hours in advance—it will keep its flavour.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.