Preheat your oven to 375ºF (190ºC).

Bring a large pot of salted water to a rolling boil. Cook the dry pasta for about 8 minutes, or until just under al dente. Drain and set aside.

In a large skillet over medium heat, melt the butter. Add the finely chopped onion and sliced cremini mushrooms, sauté for about 8 minutes until the onions are translucent.

Add minced garlic, tomato paste, and crushed red pepper flakes to the skillet, stirring well. Cook for an additional 3 to 4 minutes.

Stir in the fresh baby spinach, allowing it to wilt. Then, add the dried oregano, kosher salt, black pepper, and ground nutmeg.

Pour in the vegetable broth while scraping up any browned bits. Add the pumpkin puree and half of the grated Fontina cheese, stirring until combined.

In a large bowl, combine the cooked pasta with the pumpkin sauce mixture, stirring gently to coat.

Grease a 13x9-inch baking dish. Pour the pasta and sauce mixture into the dish and top with the remaining grated Fontina cheese and walnut-sage topping.

Cover with aluminum foil and bake for 15 minutes. Remove foil and bake for an additional 10 minutes.

Allow to cool for about 5 minutes before serving.