bring vinegar, sugar, and 1/4 teaspoon salt to a boil in a small heavy saucepan, stirring until sugar has dissolved, then boil until reduced to about 1/2 cup, 3 to 4 minutes. add cucumber-onion mixture and simmer, stirring, 2 minutes. stir cornstarch mixture, then stir into relish. simmer, stirring, 1 minute.