Preheat your oven to 375°F (190°C).

In a large pot, bring salted water to a rolling boil. Add the dry pasta and cook for about 8 minutes, or until it is just under al dente. Drain the pasta and set aside.

In a sauté pan over medium heat, melt the unsalted butter. Add finely chopped walnuts and 1 tablespoon of fresh sage, cooking for about 5 minutes until aromatic and golden.

Using the same sauté pan, heat 2 tablespoons of olive oil. Add the mushrooms and onion, cooking until softened, about 8 minutes.

Stir in the tomato paste, minced garlic, and crushed red pepper flakes. Cook for about 3-4 minutes until the tomato paste caramelizes slightly.

Add the fresh baby spinach and stir until it wilts, about 2-3 minutes. Sprinkle in the oregano, salt, pepper, and nutmeg, mixing well.

Pour in the vegetable broth and add the pumpkin puree, stirring to blend into a creamy sauce. Mix in the Parmesan and ½ cup of Fontina, cooking until smooth.

Fold the drained pasta into the pumpkin sauce, ensuring every piece is coated.

Transfer the mixture into a greased baking dish. Sprinkle remaining Fontina cheese on top, followed by the walnut-sage mixture.

Cover the dish with foil and bake for 15 minutes. Remove the foil and bake for an additional 10 minutes, or until the top is golden.