1. Place fillets in a shallow dish. Combine vinegar and 3 tablespoons water; drizzle over fillets. Cover and refrigerate 30 minutes.
2. Carefully cut rind and white pithy part from lemon using a small knife. Cut lemon crosswise into thin slices; discard seeds. Reserve 6 whole lettuce leaves; coarsely shred remaining lettuce.
3. Preheat oven to 325°.
4. Layer about 2 cups shredded lettuce, 1 1/2 cups tomato, 1 cup onion, 1 1/2 teaspoons garlic, and half of the lemon slices in a 13 x 9–inch casserole dish coated with cooking spray. Sprinkle vegetable mixture with 1/4 teaspoon salt and 1/8 teaspoon pepper. Place fillets on top of vegetable mixture. Sprinkle fish evenly with 1/2 teaspoon salt and 1/4 teaspoon pepper. Repeat layers with remaining chopped lettuce, tomato, onion, garlic, and lemon slices. Sprinkle with remaining 1/4 teaspoon salt and 1/8 teaspoon pepper. Top with reserved whole lettuce leaves; drizzle with oil.
5. Bake at 325° for 20 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness. Remove whole lettuce leaves and fillets from pan; keep warm. Return vegetables to oven. Bake an additional 20 minutes or until vegetables are tender. Place 1 whole lettuce leaf on each of 6 plates; top each serving with 1 fillet. Spoon about 1 cup vegetable mixture over each fillet.