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.

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

Place pan back on the burner to medium heat. 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 and turn brick red. Gradually add spinach, stirring continuously until greens wilt down. Stir in oregano, salt, pepper, and nutmeg.

Add vegetable broth, stirring to release any browned bits stuck to the bottom of the pan. Stir in pumpkin puree, Parmesan, and ½ cup fontina cheese; continue stirring until the fontina melts and the sauce is smooth. Add pasta and stir until all noodles are well-coated in sauce.

Transfer mixture to a 13x9-inch greased baking dish. Sprinkle remaining fontina cheese overtop, along with buttery walnut-sage mixture. Cover loosely with foil and bake for 15 minutes at 375ºF. Uncover and continue baking 10 to 15 more minutes, until the cheese is melted and golden.