Preheat oven to 350.  Melt 1/4 c. butter in a skillet and quickly sauté fillets on both sides, until just colored.  Remove to baking dish and keep warm.  Melt remaining butter in a skillet and sauté onion, garlic, and mushrooms until cooked but still firm.  Stir in flour, pepper, stock, and wine.  Cook over low heat until thickened, stir in mashed avocado and 1/2 c. cheese.  Spoon mixture over fillets and top with rest of cheese.  Bake 10 -15 minutes, or until cheese is melted.