Preheat your oven to 375ºF (190ºC). In a large pot, bring salted water to a rolling boil and add the pasta. Cook until 2 minutes shy of al dente, then drain and set aside.

In a large skillet over medium heat, melt the unsalted butter. Add the chopped walnuts and sage, toasting for about 5 minutes until golden and fragrant. Transfer to a bowl to cool.

In the same skillet, add the extra-virgin olive oil. Sauté the sliced mushrooms and chopped onion for about 8 minutes until soft.

Stir in the tomato paste, minced garlic, and crushed red pepper flakes. Cook for 3 to 4 minutes, stirring frequently.

Add the fresh spinach and stir until wilting. Season with dried oregano, kosher salt, black pepper, and ground nutmeg.

Deglaze the skillet with vegetable broth and stir in pumpkin purée, Parmesan cheese, and half of the fontina cheese. Cook until well combined.

Add the cooked pasta to the sauce and toss until coated evenly.

Transfer to a greased baking dish, spread evenly, and top with remaining fontina cheese and walnut-sage mixture.

Cover with foil and bake for 15 minutes. Remove foil and bake for an additional 10 minutes until golden and bubbly.