How to Make It
In a microwave-safe cup, combine the sugar and water. Cook at high power for 20 seconds. Stir until the sugar dissolves, then let cool.
In a small stainless steel bowl, combine the sugar syrup, Champagne and citrus juices. Set the bowl over a larger bowl of ice and keep cold. In another small bowl, combine the vinegar and shallot and season with pepper.
Arrange the shucked oysters on a bed of crushed ice and coarse salt. Add the egg-white powder to the Champagne-citrus juice mixture and beat with a handheld mixer or immersion blender until thick and frothy.
Spoon a little of the vinegar-shallot mixture over the oysters and a little of the Champagne froth on top. Garnish with the frozen Champagne grapes and serve right away.
Egg-white powder is available at health food stores and most supermarkets.