A fruit ade is sweetened with a “simple syrup,” which is made by dissolving sugar in boiling water. The syrup is important because granulated sugar would not dissolve in the cold juice. In this recipe, the sugar syrup is also the vehicle for the cool flavor of mint.
Grouper is a flavorful, meaty-textured fish and is well worth a trip to a fish market, because you’re unlikely to find this fish in supermarkets. If grouper isn’t available, substitute red snapper, sea bass, tilapia, or tilefish.