Shuck oysters into a bowl; reserve liquor for oyster stew. Arrange empty shells in a baking pan, propping them up with rock salt or extra shells. Season the shells lightly with black pepper and garlic powder; place oysters in shells and season similarly. Sprinkle the oysters with cracker crumbs. Combine the sherry and lemon juice and steep mushrooms in the mixture for a few minutes. Then spoon mushrooms onto the oysters. Top each oyster with a dollop of cheese. Dust with paprika and bake at 400 degrees F for 20 to 25 minutes. These oysters are tastiest when moist but not too juicy.