Combine onion and 2 tablespoons cilantro; set aside.
Combine salsa, broth, garlic, and 2 tablespoons cilantro in container of an electric blender; cover and process until smooth. Transfer mixture to a saucepan; bring to a boil. Reduce heat, and simmer, uncovered, 5 minutes or until slightly thickened, stirring occasionally. Remove from heat, and set aside.
Place water in a saucepan; bring to a boil. Place fish in boiling water; cover, reduce heat, and simmer 8 minutes or until fish flakes easily when tested with a fork. Remove fish with a slotted spoon; remove and discard skin. Add potato to water in saucepan; cook 5 minutes or until tender. Drain well. Flake fish with a fork. Combine fish, potato, and 1/2 cup salsa mixture; stir well.
Place tortillas on ungreased baking sheets. Bake at 350° for 15 to 20 minutes or until crisp. Place 1 tortilla on each serving plate. Top tortillas evenly with shredded lettuce, fish mixture, tomato, cheese, sour cream, and reserved onion mixture. Sprinkle evenly with remaining 2 tablespoons cilantro, and serve with remaining salsa mixture.