1. Add the oil to a large saute pan and saute all vegetables for 1 to 2 minutes. Then add the garlic, cover and sweat the vegetables on lower heat for about 2 minutes. The onion and fennel should be translucent.
2. Increase the heat to high and add mussels and Old Bay Seasoning and saute to incorporate the vegetables. When the mussels start to open, add beer and butter and cover the pan for about 4 more minutes, being careful not to overcook.
3. Remove from the heat and arrange mussels in a dish or covered mussel pot. Pour reducing liquid and vegetables on top.
4. Garnish with fennel tops, additional Old Bay Seasoning and warm crusty bread if desired.