1. From lemon, squeeze 2 tablespoons juice. In heavy 1-quart saucepan over medium heat,melt butter or margarine; stir in flour until blended; cook 1 minute. Gradually stir in lemon juice, 1 cup water, and bouillon; cook, stirring constantly, until mixture thickens slightly. Remove saucepan from heat.
2. In small bowl, beat egg yolks slightly. Into egg yolks, stir small amount of hot sauce; slowly pour egg mixture back into sauce, stirring rapidly to prevent lumping. Cook, stirring, until mixture thickens (do not boil). Keep warm.
3. Meanwhile, in 10-inch skillet, over high heat, heat frozen fish and enough water to cover to boiling. Reduce heat to low; cover and simmer 10 to 15 minutes, until fish flakes easily when tested with a fork. With slotted pancake turner, remove fish from water; drain fish (still held on pancake turner) on paper towels; place on large platter. With 2 forks, gently pull fish apart into large chunks. Spoon citrus sauce over fish.