Cook beans, covered, in small amount of boiling salted water until tender, about 15 minutes. In 2-quart saucepan, cook mushrooms and onions in butter until tender. Blend in flour, salt, thyme and pepper. Add milk, all at once. Stir constantly until mixture thickens and bubbles. Stir in beans. Heat through. Add bacon. Put hot mixture into prepared pattie shells and if desired, garnish with additional bacon. Can be served as a Main Course.