Sprinkle onion and parsley in bottom of 12x71/2x2-inch baking dish.
Arrange fish fillets in dish.
Cover; bake in a 350Â° oven for 30 minutes or till fish flakes easily when tested with a fork.
Drain off any liquid that has accumulated in baking dish.
Meanwhile, in a saucepan melt butter.
Stir in flour till blended.
Add milk all at once.
Stir in mushrooms.
Cook and stir till thickened and bubbly.
Cook and stir 1 to 2 minutes more.
Stir in sherry.
Pour over fish.
Sprinkle with Parmesan cheese.
Return to oven and bake, uncovered, about 5 minutes more or till heated through.
Sprinkle with paprika or chives.