Preheat your oven to 375°F (190°C).

In a large pot, bring salted water to a rolling boil. Add the dry pasta, cooking until 2 minutes shy of al dente, about 8-10 minutes. Drain and set aside.

Melt the butter in a large skillet over medium heat. Add the walnuts and sage, toasting them for about 5 minutes until golden and fragrant. Set aside.

In the same skillet, add olive oil and heat until shimmering. Sauté the onion and mushrooms for about 8 minutes until soft and golden.

Stir in the tomato paste, garlic, and crushed red pepper flakes. Cook for 3-4 minutes until the mixture deepens in color.

Fold in the spinach until wilted. Add oregano, salt, black pepper, and nutmeg, mixing until combined.

Deglaze with vegetable broth, scraping up bits from the bottom. Stir in pumpkin puree, Parmesan, and Fontina. Cook for 5 minutes until smooth.

Add the cooked pasta to the sauce, mixing until coated.

Lightly grease a 13x9-inch baking dish. Pour in the pasta mixture, top with remaining Fontina and the walnut-sage mixture.

Cover with foil and bake for 15 minutes. Remove the foil and bake for an additional 10 minutes until cheese is melted and golden.