Melt butter in a 4-quart saucepan on medium heat. Add onions and sauté for 5 to 7 minutes until they become translucent. Add the potatoes, peas, sorrel leaves and broth.
Bring to a boil, then reduce and simmer for 30 minutes or until the potatoes are tender. In a blender or food processor, puree soup, in batches, if necessary. Season with salt and pepper.
Divide soup into soup bowls and garnish with cream and sorrel.