Rinse oysters in cold water. Shuck oysters, reserving deep half of shells; place oysters in colander to drain. Set aside.
Combine breadcrumbs, butter, and sherry; set aside. Combine spinach, onion, seasoned salt, and Worcestershire sauce, and set aside.
Sprinkle a layer of rock salt in bottom of a large broiling pan. Place oysters in half shells; arrange shells on rock salt. Top oysters evenly with spinach mixture. Sprinkle with cheese; top evenly with breadcrumb mixture. Bake at 400° for 15 minutes. Broil 4 inches from heating element 2 minutes or until browned. Garnish with lemon wedges and parsley sprigs.