Preparation Melt butter in heavy large pot over medium heat. Add leek; sauté 3 minutes. Add mussels and wine. Cover and simmer until mussels open, about 4 minutes (discard any that do not open). Using slotted spoon, transfer mussels to 2 bowls. Stir cream and 2 tablespoons parsley into liquid in pot. Simmer uncovered 3 minutes. Season sauce with salt and pepper. Pour sauce over mussels. Sprinkle with 2 tablespoons parsley.
Preparation Melt butter in heavy large pot over medium heat. Add leek; sauté 3 minutes. Add mussels and wine. Cover and simmer until mussels open, about 4 minutes (discard any that do not open). Using slotted spoon, transfer mussels to 2 bowls. Stir cream and 2 tablespoons parsley into liquid in pot. Simmer uncovered 3 minutes. Season sauce with salt and pepper. Pour sauce over mussels. Sprinkle with 2 tablespoons parsley.