Roast the butternut squash: Preheat oven to 400°F. Place the cubed butternut squash on a parchment-lined baking sheet. Drizzle with 1 tablespoon olive oil and toss to coat. Spread into a single layer and roast for 20–25 minutes, or until tender and lightly caramelized. Remove and set aside.
Cook the sausage: Heat the remaining tablespoon of olive oil in a large skillet over medium heat. Remove the sausage from its casings and add to the skillet. Break it apart with a wooden spoon and cook until browned and cooked through, about 7–9 minutes.
Add aromatics: Add the chopped onion and cook for 3–4 minutes until softened. Stir in the garlic and fresh sage and cook for another 30 seconds until fragrant.
. Prepare the cream sauce: Pour in the chicken broth and milk. Stir to combine and bring to a gentle simmer. Add 1 cup of the grated Parmesan cheese. Stir until incorporated and allow the sauce to simmer for 3–5 minutes until slightly thickened. Season to taste.
Cook the pasta: Meanwhile, bring a large pot of salted water to a boil. Cook the rigatoni until about 2 minutes shy of al dente according to package directions. Drain, reserving a little pasta water.
Combine: Add the cooked pasta and roasted butternut squash to the sausage sauce. Gently toss until everything is coated. If the sauce seems too thick, add a splash of reserved pasta water.
Assemble: Reduce oven temperature to 375°F. Transfer the pasta mixture to a greased 9-by-13-inch baking dish. Sprinkle the mozzarella evenly over the top, followed by the remaining ½ cup Parmesan cheese.
Bake: Bake uncovered for 20–25 minutes, or until the cheese is melted, golden and bubbling around the edges. Let rest for 5–10 minutes before serving. Garnish with fresh sage if desired.