Preheat the oven to 400 degrees F.

Place the squash on a large baking sheet and drizzle with 3 tablespoons of the olive oil. Season generously with salt and pepper and then toss the squash in the oil to make sure it is evenly coated. Roast in the middle of the oven until tender and golden brown around the edges, usually about 45 minutes. Remove from the oven and set aside.

In a large skillet or a wide Dutch oven, heat the remaining tablespoon of oil over medium-high heat. Add the sausage and cook, stirring, until browned, 4 to 6 minutes. Add 2 tablespoons of the butter, the fennel, onion, and garlic and cook until vegetables are softened, about 4 minutes. Add the apple and cook for 2 minutes. Add the wine and vinegar and cook until nearly evaporated. Add the chicken stock and cook until reduced by half. Add the heavy cream, chopped marjoram and sage and cook, stirring occasionally, until sauce has thickened enough to coat the back of a spoon. Season with salt and pepper to taste. Cover and set aside to keep warm.

In a small nonstick skillet, heat the remaining butter until foamy. Add the sage leaves (in batches if necessary), and cook until lightly golden around the edges and crispy. Transfer to a small plate and repeat with any remaining leaves. Transfer the butter to a small heatproof bowl.

Add the reserved butternut squash to the sauce and stir gently to combine; take care not to break it up too much. Serve the sauce ladled over rigatoni and garnish with the crispy sage leaves. Drizzle with a bit of the reserved butter and then sprinkle with walnuts and shaved Parmesan.