Discard any open mussels that don't close when lightly touched.
Sauté onion and garlic in 1 tablespoon butter in large pot or Dutch oven over medium heat until until softened, about 3 minutes. Stir in pepper, lemon juice and wine. Bring to boiling. Add parsley.
Add mussels to pot. Cover and reduce heat to low; cook for 8 to 10 minutes or just until shells open.
Remove mussels with slotted spoon to bowl. Discard any mussels that don't open. Swirl remaining butter into cooking liquid. Spoon over mussels. Garnish with parsley. Makes 6 servings.