Preheat your oven to 400°F (200°C).

Toss the diced butternut squash with 1 tablespoon of olive oil, salt, and pepper. Spread in a single layer and roast for 25-30 minutes.

Bring a large pot of salted water to a boil and cook the pasta according to package instructions until al dente, around 8-10 minutes.

Heat the remaining 2 tablespoons of olive oil in a skillet over medium heat. Sauté the chopped onion for 5 minutes, then add the minced garlic for 1-2 more minutes.

Add the roasted butternut squash to the skillet, pour in the vegetable broth, and mash some squash for creaminess. Stir in heavy cream and Parmesan, adding reserved pasta water gradually.

Toss the cooked pasta in the skillet with the sauce, seasoning with salt, pepper, and red pepper flakes if desired.

Serve in bowls, garnished with toasted walnuts, extra Parmesan, and fresh sage leaves.