happiness is having a jar of sweet, tart, crunchy relish in the refrigerator. it's a project that easily fits into your culinary repertoire.
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. 