Ingredients : 1 pound large mushrooms 3 tablespoons butter 2 tablespoons finely chopped onion 1 (3 ounce) package cream cheese, softened 2 tablespoons prepared Dijon−style mustard 6 1/2 ounces crab meat 1/4 cup chopped water chestnuts 2 tablespoons chopped pimento peppers 4 tablespoons grated Parmesan cheese