1 lb. dry pasta of choice (such as mezzi rigatoni, shells, ziti, fusilli, or farfalle)

2 Tbsp. butter

1/2 cup finely chopped walnuts

1 Tbsp. plus 2 tsp. finely chopped fresh sage leaves, divided

2 Tbsp. extra-virgin olive oil

8 oz. cremini (baby bella) mushrooms, sliced

1 yellow onion, finely chopped

1/4 cup tomato paste

4 garlic cloves, minced

1/2 tsp. crushed red pepper flakes

3 to 4 handfuls fresh baby spinach

1 tsp. dried oregano

1 tsp. kosher salt

1/2 tsp. black pepper

1/4 tsp. ground nutmeg

3 cups vegetable broth

1 (15-oz.) can pumpkin purée

1/2 cup grated Parmesan cheese

1 1/2 cups grated fontina cheese, divided

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Preheat the oven to 375ºF. Bring a large pot of salted water to a boil. Once boiling, add pasta and cook until 2 minutes shy of al dente. Drain and set aside.
In a large sauté pan, melt butter over medium heat. Add the walnuts and 1 Tbsp. of sage, cooking for about 5 minutes, until the walnuts are toasted and fragrant. Season with a pinch of salt, and transfer the walnut-sage mixture to a bowl.
In the same pan, add olive oil, mushrooms, and onion. Cook until the vegetables are soft, about 8 minutes.
Stir in tomato paste, garlic, and red pepper flakes. Cook for 3 to 4 minutes, until the tomato paste begins to caramelize and turn a deep red.
Add spinach in batches, stirring constantly, until the spinach wilts. Stir in oregano, salt, pepper, and nutmeg, followed by the remaining sage.
Pour in the vegetable broth, scraping the bottom of the pan to release any browned bits. Stir in the pumpkin purée, Parmesan cheese, and 1/2 cup of fontina. Continue stirring until the cheese melts and the sauce is smooth.
Add the pasta to the sauce and stir to coat the noodles evenly.
Transfer the pasta mixture to a greased 13×9-inch baking dish. Sprinkle with the remaining fontina cheese and the walnut-sage topping.
Cover with foil and bake for 15 minutes. Uncover and continue baking for an additional 10 minutes, until the cheese is fully melted and golden.

For a dairy-free version, swap the cheeses for dairy-free alternatives and use olive oil instead of butter.
If you prefer a spicier dish, increase the crushed red pepper flakes or add fresh chili peppers.
This recipe can be made gluten-free by using gluten-free pasta.
To store, keep leftovers in an airtight container in the refrigerator for up to 5 days.
Feel free to add other vegetables like roasted butternut squash, zucchini, or bell peppers for extra flavor.