Process first 3 ingredients in a blender or food processor until smooth. Cover and chill.
Combine shallot and next 3 ingredients in a large bowl. Cover and chill.
Shuck oysters, reserving oyster liquor. Add oysters and liquor to vinegar mixture; cover and chill 15 minutes. Place 1 oyster in a shot glass. Dollop with cucumber sauce. Repeat procedure with remaining oysters and cucumber sauce. Garnish, if desired.
Note: If you prefer to poach the oysters, shuck them, reserving their liquid. Add oysters to boiling water in a large saucepan 3 to 5 minutes or until edges begin to curl. Drain and proceed with recipe.
