Cassis, also known as black currant, has a deep, velvety, ripe-berry flavor, along with a slight sourness; this sorbet plays up those qualities. We use bottled nectar, but if you find fresh black currants, you can certainly make your own (see cooks' note, below).
·If using fresh black currants, bring 1 cup stemmed currants to a boil with 2 1/2 cups water and 2/3 cup sugar, stirring until sugar is dissolved, then simmer, uncovered, 10 minutes. Pour through a large sieve into a bowl, pressing hard on and then discarding solids, then add crème de cassis. Chill, uncovered, until cold, 1 to 2 hours, before freezing in ice cream maker.
·We prefer Looza brand black-currant nectar, but if you can find only black-currant syrup or concentrated juice drink, which are much sweeter, omit the sugar in this recipe and use 1 cup of syrup or juice drink diluted with 2 cups water.
·Sorbet can be made 2 days ahead.
