Preheat your oven to 375ºF (190ºC). In a large pot, bring salted water to a boil, then add the dry pasta. Cook until the pasta is 2 minutes shy of al dente, about 8-10 minutes. Drain and set aside.

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

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

Stir in the tomato paste, minced garlic, and red pepper flakes, cooking for 3-4 minutes to enhance flavor.

Mix in spinach until wilted, then season with oregano, salt, black pepper, and nutmeg.

Deglaze the skillet with vegetable broth, scraping up browned bits. Stir in pumpkin purée, grated Parmesan, and Fontina, cooking until creamy.

Fold the cooked pasta into the pumpkin sauce until well-coated.

Transfer the mixture into a greased 13x9-inch baking dish. Top with the walnut-sage mixture and remaining Fontina cheese.

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