Preheat your oven to 375°F (190°C), and prepare a 13x9-inch baking dish greased with butter or oil.

In a large stockpot, bring salted water to a boil. Cook the dry pasta until just 2 minutes shy of al dente, about 8-10 minutes. Drain and set aside.

Melt 2 tablespoons of unsalted butter in a skillet over medium heat. Stir in chopped walnuts and sage, cooking until fragrant, about 3-4 minutes.

Add olive oil to the skillet and sauté sliced mushrooms and onion until soft and lightly browned, about 8 minutes.

Stir in tomato paste and minced garlic, cooking for about 4-5 minutes until the mixture caramelizes.

Add spinach to the skillet and cook until wilted, about 2-3 minutes. Season with oregano, salt, pepper, and nutmeg.

Deglaze the pan with vegetable broth, then stir in pumpkin purée and both cheeses until smooth and creamy.

Mix the drained pasta with the sauce until well coated.

Transfer to the greased baking dish, top with remaining Fontina cheese and walnut-sage mixture.

Cover with foil and bake for 15 minutes. Remove foil and bake for another 5-10 minutes until the top is golden and bubbling.