Drop flounder into boiling salted water; cook over high heat 8 minutes or until fish flakes easily when tested with a fork. Remove fish from water, and cool. Gently flake fish from bones, and set aside, discarding bones.
Sauté onion, parsley, green pepper, and garlic in butter in a large skillet; add tomato juice, tomato puree, and cooked fish, stirring well. Simmer, uncovered, 30 minutes. Stir in Mushroom Sauce, lemon slices, and wine; simmer, uncovered, an additional 10 minutes. Remove from heat, and serve immediately over hot cooked rice.