1. Place first 5 ingredients in a food processor or blender; process until slightly chunky. Transfer to a bowl; cover and chill.
2. Place onions, 1/4 cup cilantro, chile, and juice in a food processor; process until finely chopped. Add salt and fish; pulse 4 times or until fish is coarsely ground and mixture is well blended.
3. Divide fish mixture into 4 equal portions, shaping each into a 1-inch-thick patty. Cover and chill 30 minutes.
4. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Add patties to pan; cook 6 minutes on each side or until desired degree of doneness.
5. Wipe pan with paper towels; recoat with cooking spray. Place 2 buns, cut sides down, in pan; cook 2 minutes or until lightly toasted. Repeat procedure with cooking spray and remaining buns.
6. Top bottom half of each bun with 1 lettuce leaf and 1 patty. Spread about 1 tablespoon mayonnaise mixture over each patty; place top half of 1 bun on each serving.
Shape the patties up to 8 hours in advance, and cook just before serving. Use a grill pan or nonstick skillet to cook them since they may not hold up on a standard grill.