Combine first 4 ingredients in a small bowl, stirring well.
Sprinkle fish fillets evenly with salt and freshly ground black pepper. Heat a large nonstick grill pan over medium-high heat. Coat the pan with cooking spray. Add fish fillets to the pan; sauté 4 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness.
Spread 1 tablespoon mayonnaise mixture over the cut side of each bun half. Line bottom half of each bun with 1 romaine lettuce leaf half and 1 tomato slice. Top each serving with 1 fillet. Place top halves of buns on sandwiches.