Yuzu-Flavored Ponzu Sauce

Ponzu is made with the juice of a kind of citron, yuzu—a tangerine-sized citrus fruit with a thick, bumpy rind. Bright green in summer, yuzu turns golden yellow in autumn when it is ripe. Like lemon, yuzu is valued for its rind and juice, which are bursting with fragrance and a slightly tart and bitter flavor. The yuzu is not generous, producing only a little less than a tablespoon or so of juice per fruit. To get ½ cup of yuzu juice, as is suggested in this recipe, is expensive and difficu