Preheat the oven to 375°F (190°C).

In a large pot, bring salted water to a boil and cook dry pasta until just shy of al dente, about 2 minutes less than the package instructions.

In a sauté pan, melt 2 tablespoons of butter over medium heat. Add walnuts and sage, toasting for about 5 minutes until fragrant.

In the same pan, heat 1 tablespoon of olive oil over medium heat. Add diced onions and sliced mushrooms, sautéing for about 8 minutes until soft.

Stir in tomato paste, minced garlic, and red pepper flakes. Cook for an additional 3-4 minutes until the tomato paste begins to caramelize.

Gradually add spinach, stirring until it wilts down. Season with nutmeg, kosher salt, and black pepper.

Deglaze the pan with vegetable broth, scraping up any bits. Stir in pumpkin purée, grated Parmesan, and fontina cheese until smooth.

Combine the cooked pasta with the warm pumpkin sauce in a large bowl, ensuring every piece is well-coated.

Grease a 13x9-inch baking dish and transfer the pasta mixture into it. Top with remaining fontina cheese and walnut-sage mixture.

Cover with foil and bake for 15 minutes. Remove the foil and continue baking for another 10 minutes until cheese is melted and golden.