Melt butter in a large saucepan over medium heat and cool onions until soft and translucent, about 3 minutes. Add butternut squash, water, beef bouillon, marjoram, pepper, and cayenne; bring to a boil. Cook until butternut squash is tender, about 25 minutes.
Add turkey and cream cheese to the saucepan with the butternut squash mixture.
Pour soup in batches into a blender or food processor; blend until smooth. Return to saucepan and heat through, about 5 minutes, but do not let soup boil.