Preheat your oven to 375ºF (190ºC) and bring salted water to a boil in a large pot. Cook pasta until 2 minutes shy of al dente, about 8 minutes. Drain and set aside.

In a medium skillet, melt the butter and add chopped walnuts and 1 tablespoon of sage. Toast for 5 minutes until golden and fragrant. Season with salt and transfer mixture to a bowl to cool.

In the same skillet, heat olive oil and sauté chopped onion and sliced mushrooms for about 8 minutes until translucent and softened.

Add tomato paste, minced garlic, and red pepper flakes to skillet. Cook for 3-4 minutes, then fold in spinach until wilted.

Remove from heat, stir in oregano, kosher salt, black pepper, nutmeg, vegetable broth, pumpkin purée, Parmesan, and 3/4 cup Fontina cheese until creamy.

Combine the cooked pasta with the veggie-pumpkin sauce until thoroughly coated.

Pour mixture into a greased baking dish, sprinkle remaining Fontina cheese on top and add walnut-sage topping. Cover with foil and bake for 15 minutes, then uncover and bake for 10 more minutes until golden brown.