Combine fruit juices in pitcher. According to taste, fill glass with about 1/3 juice to 2/3 seltzer, allowing room for sorbet. Scoop a generous amount of lemon sorbet on top. Garnish with a sprig of mint.
Using white grapefruit juice will lend some bitterness to the combination that is offset by the sorbet. Alternatively, use ruby red grapefruit juice, which is sweeter.