Our favorite spot for a casual lunch in the Florida Keys is on the bay side at Lorelei. Their grilled fish sandwich is our standing order when we visit. This grilled mahi mahi sandwich is the closest thing you can get without a plane ticket.
Pat fish fillets dry and season with salt and pepper.
Preheat the grill to about 450° - 500°. Use a grill brush to clean the grill grates well. Fold a paper towel about 3-4 times and pour about 2 to 3 teaspoons of oil onto the towel. Rub the cleaned grill grates quickly with the oiled towel.
Place the fish fillets onto the grill and close the lid. Cook for 3-4 minutes on one side and slide a grilling spatula quickly and with one motion under the fish. Flip the fish and cook on the other side for an additional 2-4 minutes (depending on the thickness of your fish fillets).
During the last 1 1/2 to 2 minutes, open the buns and place them cut side down to grill. They should take about 1-2 minutes to get grill marks.
Transfer the fish to a cutting board or rimmed baking sheet.
Place lettuce leaves, slices of tomato and rings of red onion (if using) on the bottom portion of the bun (this prevents juices from the fish leaking into the bottom bun and getting soggy). Add the grilled fish fillet. Slather tartar sauce onto the top portion of the bun and place it on the sandwich. Enjoy!