Preheat your oven to 375°F (190°C) and prepare your baking dish by lightly greasing it.

In a large pot, bring salted water to a rolling boil, add dry pasta, and cook until 2 minutes shy of al dente, about 7-9 minutes. Drain and set aside.

In a skillet, melt 2 tablespoons of butter over medium heat, add chopped walnuts and 1 tablespoon of fresh sage, cooking for 3-4 minutes until golden. Set aside.

In the same skillet, heat 2 tablespoons of olive oil, add minced garlic, diced mushrooms, and chopped onion. Sauté for about 8 minutes until softened.

Stir in the tomato paste and crushed red pepper flakes; cook for 3-4 minutes until caramelized and aromatic.

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

Pour in vegetable broth, deglaze the pan, then stir in pumpkin purée and both cheeses until smooth.

Combine the drained pasta with the sauce, ensuring all pasta is well-coated.

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

Cover with foil and bake for 15 minutes, then remove foil and bake for another 10 minutes until golden and bubbly.

Let the pasta bake rest for about 5 minutes before serving.