Bring water to a boil in a saucepan. Loosen oysters from shells; add oysters to boiling water. Simmer 3 to 5 minutes or until oysters are plump and opaque; drain. Return oysters to shells. Discard top shells, keeping oysters in the deeper bottom shells.
Sprinkle a thin layer of rock salt in a jelly roll pan. Arrange oysters in shells over rock salt; set aside.
Heat oil in a skillet over medium heat. Add onion; cook 2 minutes, stirring often, until onion is soft. Add artichoke hearts and spinach; sauté 2 minutes. Stir in Greek seasoning and cream cheese.
Top each oyster with 1 heaping tablespoon of spinach mixture; sprinkle oysters evenly with Parmesan cheese. Broil oysters 4 minutes or until cheese melts.