Use a serrated knife to trim off the ends and the peel from the grapefruits.

Working over your serving or storage bowl, carefully cut away the grapefruit segments, leaving the connective pith, dropping them into a bowl as you cut.

Trim off the top of the fennel bulb then shave the fennel using a mandoline cutter on the thinnest setting. You can also use the slicer insert on a food processor or a very sharp knife. The thinner, the better.

Add the mint leaves (no stems) to the bowl and toss with the remaining ingredients.