Wash and drain green beans. Break stem ends off green beans and cut into 2 inch pieces.
Melt butter over medium heat in 10" to 12" skillet. Add onions and garlic and cook until onions are tender (approximately 5 minutes).
Stir in mushrooms, fully coating with the butter and onion mixture. Add beans and sherry (or water); sprinkle with salt, pepper and thyme.
Bring mixture just to a boil. Reduce heat and simmer covered for 10-15 minutes or until beans are tender yet crisp.