In 2-cup glass measure, mix half 'n half, sherry and tarragon.
Heat at high1 minute.
Add instant soup mix to hot half 'n half mixture; stir well.
Place butter in 2-quart oblong baking dish and heat at high1 minute.
Place fillets in dish so that thickest portion of fillets is towards edge of dish.
Pour mushroom sauce over fish.