Scrub oysters under cold running water. Spread rock salt on a heavy roasting pan or baking sheet. Arrange one dozen of the oysters, curved shell down, on rock salt so the oysters are stabilized.
Bake in a 400 degree F oven for 10 minutes or until the top shells just begin to open. Remove oysters as they pop and continue baking until all are done. Repeat with remaining oysters.
Hold the roasted oyster in an oven mitt with the flat side up. Using a strong-bladed knife with a hand guard, insert the knife tip into the hinge between the shells. Twisting the blade to pry open the oyster, move the blade along the inside of the upper shell to free the muscle from the shell. Remove and discard the top shell of the oyster. Slide the knife under the oyster to sever the muscle from the bottom shell. Use this shell for serving the oyster on the half shell. Discard any bits of shell on the oyster. Transfer to a platter lined with rock salt. Top each oyster with 1 teaspoon each tomatoes and green onions. Serve with lemon wedges. Makes 24 appetizer servings.