1. Rinse flounder fillets and pat dry with paper towels. Brush fillets with lemon juice. In small bowl, combine tarragon, 1/2 teaspoon salt, and 1/8 teaspoon white pepper; sprinkle evenly on fillets. Fold fish fillets into thirds; arrange folded fillets in 8" by 8" baking dish. Sprinkle grapes and 1 tablespoon white wine over and around fish. Cover dish with waxed paper; set aside while preparing sauce.
2. In small bowl, place butter or margarine, flour, 1/2 teaspoon salt, and 1/8 teaspoon white pepper; cover with large plate. Cook at high (100% power) 1 minute or until butter or margarine melts; stir until smooth. Stir in half-and-half until blended. Cook at high 3 minutes, stirring after 2 minutes, until sauce thickens and boils. Stir in remaining 2 tablespoons wine.
3. In another small bowl, beat egg yolk slightly; stir in small amount of hot sauce. Slowly pour egg mixture back into sauce, stirring rapidly to prevent lumping. Cook at medium (50% power) 1 minute, stirring after 30 seconds, or until sauce thickens slightly. Cover and keep sauce warm.
4. Cook fish at high 5 to 7 minutes just until fish flakes easily when tested with a fork, rotating dish a half turn after 4 minutes.