Preheat oven to 425°F. Line a baking sheet with parchment paper and drizzle squash with olive oil. Roast, cut-side down, to tender, about 45 minutes to 1 hour. Cool to handle and remove seeds. Scrape flesh from squash skin and mash in large bowl; season with salt, pepper and nutmeg to taste. Combine squash with ricotta cheese.
Reduce oven heat to 375°F.
Bring a large pot of water to a boil for the pasta. Salt water and cook pasta 5-6 minutes, undercooking it by about 2-3 minutes.
Heat a small skillet over medium heat and melt the butter. When foam subsides, crisp sage leaves and brown the butter until nutty and fragrant. Remove the sage leaves to a paper towel-lined plate to drain. Add butter and crumbled sage to squash and ricotta mixture.
Heat a skillet over medium-high heat. Brown and crumble the sausage into small bits and add garlic and fennel. Add sausage to squash and combine. Add half the stock to thin the mixture a bit then add pasta and half the grated cheese. If mixture is thick and doesn’t move freely when shaken, add more stock. The dish should look like it’s covered in thick sauce. Transfer pasta to a casserole dish and top with remaining cheese. Bake until brown and bubbly, 20-30 minutes.
Serve with salad of escarole, Romaine and shaved fennel dressed with a simple balsamic or lemon vinaigrette.