Blend egg yolks in a food processor, then add vinegar and mustard and blend again.
Slowly drizzle in the oil while blending until the mayonnaise is thick.
Stir in half the grapefruit zest and the crab meat, then set aside.
Whisk olive oil with the reserved grapefruit juices, remaining zest, and mustard to make the dressing.
Preheat the grill to high and grill mackerel skin side-up for 2 to 4 minutes.
Blanch asparagus in boiling water for 1 to 2 minutes, then refresh in iced water.
Sauté the cooled asparagus in a hot pan with butter for 1 to 2 minutes until colored.
Plate the asparagus and mackerel, top with mayonnaise, and decorate with fruit segments.
Drizzle with citrus dressing and garnish with watercress to serve.