Preheat oven to 350°.
Place fillets on a baking sheet lightly coated with cooking spray. Bake at 350° for 16 minutes or until fish flakes easily when tested with a fork. Cool 5 minutes.
Flake fillets with a fork into 1/2-inch pieces. Add onion and next 7 ingredients; stir to combine. Cover and chill 2 hours.
Place a lettuce leaf on each of 6 serving plates; top each leaf with 2/3 cup salad.