Scrub mussels and pull off beards, cutting them at the base with a paring knife. Discard those that do not close when you handle them and any with broken shells.
Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain and reserve.
Meanwhile, in a large frypan place 2 tablespoons butter or margarine, onion, garlic, parsley, bay leaf, thyme and wine. Bring to a boil and lower heat; cook for 2 minutes. Add mussels; cover and cook just until they open, 3 to 4 minutes. Do not overcook.
Divide pasta into 4 bowls and spoon mussels over pasta.
Strain mussel liquid and return to pot. Add remaining butter or margarine and heat until it melts. Pour over mussels and serve.