Cook the pasta according to package directions, approximately 10–12 minutes. Drain and set aside.
Heat olive oil in a sauté pan over medium-high heat. Add garlic and cook for 2–3 minutes until fragrant.
Lower heat to medium. Add pumpkin purée, milk, vinegar, broth, salt, and pepper. Whisk and simmer for 4–5 minutes.
Whisk in sage and Parmesan. Turn off the heat and continue whisking for 2–3 minutes, until slightly thickened.
Stir in spinach until wilted. Add cooked pasta and toss until coated.
Serve with walnuts, pepitas, extra Parmesan, and sage if desired.