Preheat your oven to 375°F (190°C). In a large pot, bring heavily salted water to a boil, add the dry pasta and cook until just 2 minutes shy of al dente, about 7-8 minutes. Drain and set aside.

In a large skillet over medium heat, melt the unsalted butter. Add the finely chopped walnuts and 1 tablespoon of fresh sage. Toast for about 5 minutes.

Using the same skillet, add extra-virgin olive oil. Toss in the sliced cremini mushrooms and finely chopped yellow onion. Sauté for about 8 minutes.

Stir in the tomato paste, minced garlic, and crushed red pepper flakes into the skillet. Sauté for 3-4 minutes.

Gradually mix in the fresh baby spinach until it wilts down, about 2-3 minutes. Incorporate remaining sage, oregano, salt, black pepper, and nutmeg, then add the vegetable broth and pumpkin puree.

Add the cooked pasta into the skillet, ensuring every piece is well-coated. Taste and adjust seasoning if necessary.

Transfer to a greased 13x9 baking dish. Sprinkle half of the grated Fontina cheese over the top.

Cover the dish tightly with aluminum foil and bake for 15 minutes. Remove the foil and sprinkle remaining Fontina cheese, then bake uncovered for another 10 minutes.