Preheat oven to 375ºF.

Bring a large pot of generously salted water to a boil. Once boiling, add pasta and cook until 2 minutes shy of al dente. Drain the pasta into a strainer and set aside.

Melt butter in a large sauté pan over medium heat. Add the walnuts and fresh sage; cook 5 minutes until toasted and aromatic. Season with a pinch of salt, and transfer mixture to a bowl.

Add olive oil, along with mushrooms and onion. Cook until soft, about 7 to 8 minutes.

Stir in tomato paste, garlic, and red pepper flakes. Cook 2 to 3 minutes, until the tomato paste begins to caramelize. Gradually add spinach, stirring continuously until greens wilt down. Stir in oregano, salt, pepper, and nutmeg.

Add vegetable broth, stirring to release browned bits stuck to the bottom of the pan. Stir in pumpkin puree, Parmesan, and ½ cup fontina cheese until melted and smooth. Add pasta and stir until well-coated.

Transfer mixture to a greased 13x9-inch baking dish. Sprinkle remaining fontina cheese and walnut-sage mixture over top. Cover loosely with foil and bake for 15 minutes. Uncover and continue baking 10 to 15 more minutes, until the cheese is melted and golden.