Grouper is a white-meat fish that is well suited for the grill. If you can't find grouper, use sea bass or mahimahi. Look for fish that is free of blemishes, has a fresh smell, and has flesh that springs back when touched. Serve these fish sandwiches with fresh seasonal cherries and pineapple to add bright color and tart flavor to your meal.
Brush fillets evenly with olive oil; sprinkle evenly with salt and pepper. Place fillets on grill rack coated with cooking spray; grill 3 to 4 minutes on each side or until fish flakes easily when tested with a fork.
While fish cooks, prepare Tartar Sauce. Spread about 2 1/2 tablespoons Tartar Sauce over cut sides of each bun; place 1 lettuce leaf on bottom half of each bun. Top lettuce with fish; top with remaining bun halves.