Preheat your oven to 400°F (200°C).

Peel and cube the butternut squash into 1-inch pieces. Spread on a baking sheet, drizzle with olive oil, salt, and pepper, and toss to coat. Roast for 25–30 minutes until golden brown and fork-tender.

Bring a large pot of salted water to a boil. Cook the pasta according to package instructions, usually around 8–10 minutes. Reserve 1 cup of pasta water, then drain.

In a large skillet, heat remaining olive oil over medium heat. Sauté chopped onion for 5 minutes until translucent, then add garlic, sage, and red pepper flakes. Cook for 1-2 minutes until fragrant.

Add roasted butternut squash and vegetable broth. Mash some squash for texture, then stir in heavy cream and Parmesan. Gradually add reserved pasta water to reach desired consistency.

Toss the cooked pasta in the sauce, cooking for another minute over low heat. Adjust seasoning with salt and pepper.

Serve immediately, garnishing with toasted walnuts and fresh sage leaves.