1. Wash the mussles in cold water before cooking.
2. Heat the butter in a large skillet and saute the vegetables, onion and garlic for about 2 minutes at high temperature.
3. Add the mussels and bacon bits and keep cooking until the mussels start opening.
4. Add the Guinness beer and fresh thyme and cover the skillet with a lid.
5. Steam the mussels until all of them are open.
6. Discard any mussels that do not open at all in 3 minutes.
7. Uncover and cook for about 2 minutes more until most of the moisture evaporates.
8. Season the mussels to taste and serve them in a large bowl.
9. Sprinkle with some freshly chopped parsley.