Preheat oven to 350. Melt 3 TBSp of butter. Place halibut in baking pan and pour melted butter over fillets. Sprinkle with salt and pepper. Bake for 20 to 30 minutes until fish is flakey.
While fish is baking, melt 3 TBSP butter in skillet. Add sliced mushrooms and saute until golden. Add green onions and continue to saute for three minutes. Add flour and saute until flour has coated mushrooms and onions. Add sherry and chicken bouillon. Continue to stir and add milk. As it thickens add cajun seasoning and salt & pepper to taste. Bring to simmer and continue stirring until desired thickness.
Transfer halibut onto serving dish. Top each fillet with enough sauce to cover. Serve remaining sauce at the table for additional servings.