Preheat oven broiler and coat the broiler pan with a little olive oil.
Brush both sides of fillets with lemon juice, sprinkle with salt and pepper and place on prepared broiler pan, flat sides up.
In a small bowl, combine Parmesan cheese, butter and mayonnaise; set aside.
Broil fish about 4 inches from heat source for 6 minutes then turn and spread cheese mixture evenly over each fillet.
Top with tomato slices and broil for 4 to 6 minutes or until fish flakes easily when tested with a fork.