Sauté the shallots in the butter over low heat. Do not brown. Stir in the flour and then the fish stock, simmer about 5 minutes and add the shrimp, mushrooms, salt, and cayenne. Continue to cook until thickened. Place this stuffing onto one half of the fish and cover with the other half, reserving a little to use as a sauce. Place in a buttered ovenproof dish and bake, uncovered, in a preheated 400ºF. oven about half an hour, or until flesh flakes when fork-tested. Place on a heated platter, spoon the remaining stuffing, and garnish with parsley sprigs. 4 servings. If a lean fish is substituted (Pollock, red snapper, haddock, cod, etc.) fish should be brushed generously with oil before baking.