Season yellowtail with salt & pepper on both sides, and place on hot grill. Grill to medium (3 minutes per side depending on thickness), as overcooking will lead to a dry finished product. While fish is cooking, place all ingredients for the pineapple salsa in a bowl and toss. When the fish is cooked, place a piece of banana leaf on the plate. Place fish on top of banana leaf and top with pineapple salsa, serve with rice and garnish with a grilled lemon.
Salsa can be made 1-2 hours in advance, no need to chill.