1. Fold each flounder fillet crosswise in half. In 10-inch skillet over high heat, heat 2 cups water and 1/4 teaspoon salt to boiling. Place folded fillets in skillet; heat to boiling. Reduce heat to low; cover; simmer 5 to 7 minutes until fish flakes easily when tested with fork.
2. Meanwhile, prepare sauce: In double boiler over hot, not boiling, water, with wire whisk, beat egg yolk, lemon juice, and 1 tablespoon water until mixture thickens slightly. Add butter or margarine, 1 tablespoon at a time, beating constantly with wire whisk until butter melts and mixture thickens. Stir in capers, pars ley, and 1/8 teaspoon salt.
3. Arrange fish on platter. Pour sauce over fish.