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Top and tail the grapefruits, then remove the skin and pith with a knife. Holding each grapefruit over a bowl, cut into segments and squeeze any juice from the remaining skin and pith into the bowl.
To make the dressing, mix the grapefruit juice with the honey, mustard and oil. Season to taste.
Toss the grapefruit segments with the fennel, onion, watercress and half the dressing. Arrange on a serving plate, flake over the salmon, drizzle with the remaining dressing and scatter over the crushed fennel seeds.