Preheat your oven to 375°F (190°C).

Cook pasta in boiling salted water until al dente, about 8-10 minutes. Drain and set aside.

Toast walnuts with butter and sage in a skillet over medium heat for about 5 minutes until golden.

Sauté sliced mushrooms and onions in the same skillet until tender, about 7-8 minutes.

Add tomato paste, garlic, and red pepper flakes to the sautéed vegetables. Cook for 2-3 minutes.

Stir in spinach until wilted, then mix in vegetable broth, pumpkin purée, Parmesan, and half the fontina. Stir until smooth.

Combine cooked pasta with sauce and transfer to a greased baking dish. Top with remaining fontina and walnut-sage mixture.

Cover with foil and bake for 15 minutes. Remove foil and bake for additional 10-15 minutes until golden.

Garnish with sage or walnuts before serving.