Step 1: Using a strainer, drain oysters or shuck oysters. Save the liquid (or liquor) from inside the container or shells.
Step 2: Slice leeks into very thin pieces.
Step 3: Heat oyster liquor in a large saucepan and whisk in cubes (one at a time) of butter over very low heat. Wait until each cube is melted before adding the next one. When all of the butter has been added, you should have a nice rich sauce.
Step 4: Add leeks and oysters and gently heat until oysters are warmed through. Do not allow butter to boil.
Step 5: To serve, divide oysters into 6 small bowls. Add an equal amount of leeks and butter sauce to each. Garnish with a generous spoonful of caviar. Serve immediately as a beautiful appetizer. Bon appètit!