Melt butter in a large pot over medium-high heat. Cook and stir butternut squash, sweet potato, carrot, celery, onion, and garlic in hot butter until lightly browned, about 5 minutes.
Pour chicken stock and sweet bell peppers into butternut squash mixture; bring to a boil. Reduce heat to low, cover, and simmer until squash and potato are tender, about 40 minutes.
Pour squash mixture into a blender no more than half full. Cover and hold lid down; pulse a few times before leaving on to blend. Puree in batches until smooth.
Return pureed soup to pot and season with salt and black pepper.
