Preheat your oven to 375ºF (190ºC).

Bring a large pot of salted water to a rolling boil. Add the dry pasta and cook for 2 minutes less than the package instructions suggest. Drain the pasta and set it aside.

In a large skillet over medium heat, melt the unsalted butter. Add the finely chopped walnuts and sage, stirring until lightly toasted.

In the same skillet, add 2 tablespoons of olive oil over medium heat. Sauté the sliced mushrooms and chopped onion until they are soft and golden.

Stir in the tomato paste, minced garlic, and crushed red pepper flakes into your sautéed vegetables. Cook until the tomato paste caramelizes slightly.

Gradually add the baby spinach in batches, allowing it to wilt down before proceeding. Stir in the dried oregano, salt, black pepper, and nutmeg.

Pour in the vegetable broth and deglaze the skillet, scraping any tasty bits from the bottom. Add the pumpkin purée and half of the grated fontina, stirring until smooth.

Add the cooked pasta to the creamy pumpkin tomato sauce, folding gently to coat everything evenly.

Pour the pasta mixture into a greased 13x9-inch baking dish. Distribute the remaining fontina cheese on top, followed by the walnut-sage mixture.

Cover with foil and bake for 15 minutes, then remove the foil and bake for another 10 minutes until the cheese is golden.