Preheat your oven to 375°F (190°C). Bring a large pot of salted water to a rolling boil and cook the dry pasta until just 2 minutes shy of al dente, about 7-8 minutes. Drain the pasta and set aside.

In a large skillet, melt 2 tablespoons of butter over medium heat. Add the finely chopped walnuts and fresh sage leaves, toasting for about 5 minutes until golden and fragrant. Set aside.

Using the same skillet, add 2 tablespoons of extra-virgin olive oil. Add the sliced cremini mushrooms and chopped yellow onion, sautéing for about 7-8 minutes until onions are translucent and mushrooms are soft.

Stir in 1/4 cup of tomato paste, 4 minced garlic cloves, and 1/2 teaspoon of crushed red pepper flakes, cooking for 2-3 minutes until the tomato paste caramelizes slightly.

Add the fresh baby spinach to the skillet, cooking until it wilts down, about 2 minutes. Season with 1 teaspoon of dried oregano, 1 teaspoon of kosher salt, 1/2 teaspoon of black pepper, and 1/4 teaspoon of ground nutmeg.

Deglaze the skillet with 3 cups of vegetable broth, scraping any flavorful bits from the bottom. Add the pumpkin purée, 1/2 cup of grated Parmesan cheese, and 3/4 cup of grated fontina cheese, cooking over low heat for 3-5 minutes until smooth and creamy.

Fold the cooked pasta into the pumpkin sauce until well-coated. Transfer to a greased 13x9-inch baking dish, spreading evenly. Sprinkle remaining 3/4 cup of grated fontina cheese on top, followed by the toasted walnut-sage mixture.

Cover the baking dish with aluminum foil and bake for 15 minutes. Uncover and bake for an additional 10-15 minutes until the top is golden and bubbly.