Wash fish and dry on paper towels. Sauté shallots and mushrooms in ¼pound butter for 3 minutes. Put parsley, crumbs, and shrimp into large mixing bowl. Add mushroom mixture. Toss, taste, and adjust seasonings, then stuff into dry fish cavity. Sew up cavity with needle and thread, or close with skewers.
Cover broiler rack with foil. Coat fish with melted butter and dust with flour. Set fish, belly down, on buttered foil. Cover tail with foil. Bake in 400-degree oven for about 1 hour and 15 minutes. Drizzle with melted butter during baking time. (Butter is better than salt pork when the fish contains stuffing to be eaten.)
When done, lift fish and foil to hot platter and tear out foil. Remove string or skewers. Be careful not to damage fish. Decorate platter and serve with Champignon sauce.
To carve the fish. Cut along the top, or dorsal fin, remove fins, and skin the fish. With a fork carefully pull the fillet away from the backbone. Serve dressing from cavity with each portion.