Gently sauté mushrooms in butter until they have absorbed all their water and begin to soften. Do not brown.
Mix corn starch into cream, then add cream and tarragon to the pan and let simmer for 4-5 minutes until it thickens.
Cover and set aside. ( Thin with milk if too thick)
Place fish in deep sauté pan.
Add enough water to cover the fish completely.
On medium to high heat, bring to a gentle simmer – reduce heat, cover and cook for about 6-8 minutes.
Test fish for doneness (it should flake apart at ridges, and be opaque).
Drain water from the pan.
Slide fillets on the serving plate.
Spoon warm mushroom sauce on top and serve immediately.