1. With a sharp knife, cut the top and bottom off each grapefruit. Cutting strips from top to bottom, cut away the rind and membrane, cutting all the way down to the flesh. Once the grapefruit are peeled, cut the sections free, carefully slicing along the partitioning membranes.
2. Put the grapefruit sections in a bowl and squeeze in the juice from the remaining membrane and pith. Measure out 2 tablespoons grapefruit juice from the bowl and mix with the vinegar. Season with salt and a generous grind of pepper and whisk in the olive oil. Taste and adjust the salt and acid as needed.
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