Melt the butter in a soup pot over medium heat, then add the onion, salt, and pepper, and cook for 10 minutes, until the onions are soft. Add roasted butternut squash and stir for few minutes.

Add the stock, bring the liquid to a boil over high heat, then reduce to a simmer. Cook for 2 minutes at a simmer.

Puree the soup, either using an immersion blender, or by cooling the soup slightly and blending in a blender.

Serve as is, or with cheese and a few pumpkin seeds, if desired.