In the same skillet used for the sausage, sauté the diced onion until translucent.
Add minced garlic and cook for another minute.
Add the roasted butternut squash, chicken broth, and heavy cream.
Use an immersion blender to puree the mixture until smooth. If you don't have an immersion blender, carefully transfer to a standard blender.
Stir in the Parmesan cheese and chopped sage.
Season with salt and pepper to taste.
The key to a perfect sauce is achieving the right consistency – it should coat the back of a spoon but still be pourable. If it's too thick, add a bit more broth; if too thin, let it simmer to reduce.