1 Peel 1 grapefruit, taking care to remove white skin and loosen fruit segments from partitions. Cut grapefruit into small cubes and mix with crab. Trim mushrooms, brush and cut into small slices. Mix mushrooms in a bowl with mayonnaise, creme fraiche, tomato puree and chopped watercress. Add grapefruit, crab and shrimp and season with salt, pepper and sugar.
2 Cut remaining grapefruits in half. To serve, arrange a few lettuce leaves on a plate, place grapefruit halves on top and cover with grapefruit shrimp salad. Garnish with garden cress.