1. Bring vinegar, sugar, hot-pepper flakes, and 2 tablespoons water to a simmer in a small saucepan. Remove pan from heat and let cool. Stir in cucumber and chill, covered, stirring occasionally, at least 20 minutes.
2. Arrange half of cleaned and dried oyster shells on a bed of coarse salt. Place an oyster in each with some of the oyster liquid. Serve pickled cucumber on the side of spooned on top of each oyster, and garnish with chervil. Makes 2 servings.