Heat a barbecue or char-grill pan to high heat. Place a rondeau or wide shallow saucepan on the barbecue and heat until hot. Add oil, onion, bacon, thyme and garlic, and stir until onion is tender and bacon browns (3-5 minutes). Add beer and bring to a simmer, then add mussels, close the barbecue lid or place a lid on pan and cook, shaking pan occasionally, until mussels open (4-8 minutes). Stir in butter and parsley and serve.