Combine first 8 ingredients in a large stockpot; stir well. Add mussels; cover and cook over high heat 13 minutes or until mussels open, stirring well after 3 minutes. Discard thyme, bay leaves, and any unopened shells. Remove mussels with a slotted spoon, and divide into 5 individual shallow bowls.
Combine flour and margarine in a small bowl; stir well. Add flour mixture to wine mixture, stirring with a wire whisk until blended. Bring to a boil, and cook 8 minutes or until slightly thickened. Spoon wine mixture over mussels. Serve with French bread.