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. 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.

Wash the rocket and dry well. Cut off the root end from the onion and trim away the tough dark green part of the stalk. Cut the onion in half lengthwise and remove any dirty outer layers. Cut the onion into thin half-moons, and marinate them in a small bowl, tossed with 1 tablespoon of the dressing. Cut the avocados in half, remove the pits, and gently scoop out the flesh with a large spoon. Place the halves, cut side down, on a cutting board and cut lengthwise into ¼-inch-thick slices. Toss the rocket with 1 tablespoon of the dressing. Make a nest of rocket on a serving platter, arrange the avocado slices and grapefruit segments on it, and spoon over the remaining dressing. Garnish with the marinated onion and serve.