In a medium-sized, heat-resistant, non-metallic skillet. melt butter in Microwave Oven 30 seconds. Add chopped onion and heat, uncovered, in Microwave Oven 3 minutes, or until onion is soft.
Add tarragon leaves and mushrooms to onions and heat. covered, in Microwave Oven 3 minutes.
In a small bowl, combine sour cream, light cream, paprika. salt and flour. Add to onion-mushroom mixture. Heat, uncovered. 2 minutes in Microwave Oven. Stir. Heat, uncovered, an additional minute in Microwave Oven.
In a buttered, 3-quart, heat-resistant, non-metallic baking dish, pour half of the mushroom sauce. Arrange fillets on top. Spoon remaining mushroom sauce over fish. Sprinkle with parsley flakes.
Heat, uncovered, in Microwave Oven 7 minutes, or until fish flakes easily with a fork.