Heat oil and melt butter in a large pot over medium heat. Cook and stir onion in the butter and oil under tender.
Add squash and carrots into the pot. Pour in vegetable stock and season with salt, pepper, and nutmeg. Bring to a boil, reduce heat, and simmer until vegetables are tender.
Puree soup mixture in a blender or food processor until smooth. Return to the pot and stir in heavy cream. Heat through but do not boil. Serve warm with a dash of nutmeg.
