In a soup pot heat the oil, add the carrots and leeks. Cook for 2 minutes then add the garlic, tomatoes and wine, bring to a boil and throw in the mussels. Cover and steam until they open. Remove the mussels and add the water and parsley. Simmer for 4 minutes, meanwhile picking the mussels from the shell. Add the cream to the broth and season. Add the mussels back into the soup. Stir in the butter right before you serve. Garnish with the stuffed mussels from previous recipe.