Preparation Toss cucumber and onion with 3/4 teaspoon salt in a sieve set over a bowl and drain 3 hours. Discard liquid from bowl. Wrap cucumber and onion in a kitchen towel (not terry cloth) and squeeze out as much liquid as possible. 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. Transfer relish to a bowl and chill, uncovered, until cold, about 1 1/2 hours. Cooks' note:Relish keeps, chilled, 1 month.
Preparation Toss cucumber and onion with 3/4 teaspoon salt in a sieve set over a bowl and drain 3 hours. Discard liquid from bowl. Wrap cucumber and onion in a kitchen towel (not terry cloth) and squeeze out as much liquid as possible. 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. Transfer relish to a bowl and chill, uncovered, until cold, about 1 1/2 hours. Cooks' note:Relish keeps, chilled, 1 month.