To section the grapefruits or oranges, cut the peel from the top and bottom and stand each upright on a cutting board. Cut down from top to bottom along the peel to remove it and expose the flesh. Cut along each side of the membranes to separate the segments, and place the segments in a large bowl, along with any juice that you can squeeze out of what remains (sometimes it helps to carefully cut the grapefruit directly over a bowl). Set aside a few tablespoons of the juice to mix with the oil.

In a small bowl, whisk the oil with the lime juice, a few tablespoons of the grapefruit juice, and a generous pinch of sea salt. Place the sliced avocado in a bowl and pour some of the dressing over it, tossing very gently to coat.

Add the fennel, the remaining dressing, and the mint to the grapefruit and toss well. Gently combine the avocado with the grapefruit and fennel and divide among serving plates. Season with salt and pepper to taste, sprinkle with cracked coriander, if using, and garnish with fennel fronds.