Preheat grill and granite slab to medium high heat.
Melt 2 tbsp. of butter in a pan over medium high heat and add the red pepper, leek and onion. Sauté for 4 minutes. Stirring occasionally, add the tomatoes and the beer and bring to a boil. Reduce heat and simmer for 5 minutes to reduce the liquid by half. Stir in the mussels and cook for 1 minute. Add the green onions, season with salt and pepper and remove from heat. Whisk in the ¼ cup of butter and set aside in a warm place in the kitchen.
Season the grouper fillet with lemon pepper and oil, rubbing the seasoning into the flesh.
Reduce the heat of the grill to medium low and place the grouper on the granite slab. Close the lid and bake for 20 to 30 minutes depending on your grill until fully cooked, moist and the meat is fork tender. Squeeze the juice from the lemon over the fish and carefully remove the fish from the grill. Serve with mussel and beer butter sauce.