Heat oil in a large skillet over medium heat. Add onion and fry, stirring occasionally, until translucent, 3-5 minutes. Add garlic, salt, and pepper and continue cooking until garlic is fragrant, 1 minute more. Add squash and sage and about 1 cup water. Cover, reduce heat as needed, and simmer until squash is tender, 30-45 minutes.

Blend until smooth, tasting and adjusting salt, pepper, and sage as needed. Return to a clean skillet over low heat.

Bring a large kettle of salted water to a boil. Add pasta and cook until still slightly chewy. Reserve about 1 cup of the pasta water before draining thoroughly.

Add pasta to the sauce, tossing to coat, and adjust thickness of the sauce with the reserved pasta water.

Divide between bowls and serve with a generous amount of grated Parmesan and freshly ground black pepper.