Preheat your oven to 375ºF (190ºC).

In a large pot, bring enough salted water to a rolling boil. Add your tubular pasta and cook according to the package instructions, stopping 2 minutes shy of al dente. Drain and set aside.

In a skillet, melt 2 tablespoons of butter over medium heat. Add chopped walnuts and 1 tablespoon of sage, toasting for about 5 minutes until golden and fragrant. Transfer to a bowl and set aside.

Add 2 tablespoons of olive oil to the skillet, then toss in the sliced mushrooms and onion. Sauté for about 8 minutes until softened and caramelized.

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

Gradually incorporate the spinach, allowing it to wilt. Stir in oregano, salt, pepper, and nutmeg, mixing thoroughly.

Deglaze the skillet with vegetable broth, then stir in the pumpkin purée until smooth. Mix in Parmesan and ½ cup of grated Fontina cheese until creamy.

Add the drained pasta to the skillet, tossing until well coated with the creamy sauce.

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

Cover with aluminum foil and bake for 15 minutes. Uncover and bake for another 10 minutes until the cheese is melted and golden brown.