In a small bowl whisk together the rice wine vinegar, shallots, ginger, and cucumber. Cover and chill for at least 1 hour or the day before.
Shuck the oysters. Tip out the oyster juice into the mignonette sauce. Cut the shiso leaves in half lengthwise and slip them under the oysters. Spoon the cucumber mignonette on top. Serve on a bed of crushed ice.
Edited: October 9th, 2009