If necessary, let the prawns thaw. Peel the grapefruit so that the white skin is completely removed. Remove the fillets with a sharp knife from between the separating skins. Squeeze the juice out of the separating membranes, collect it. Clean the chilli, cut lengthways, core, wash and cut into thin rings. Mix together grapefruit juice, vinegar, chilli, salt, pepper and a pinch of sugar. Fold in 3 tablespoons of oil.
Sort out the lettuce, wash and drain. Halve the avocados lengthways and remove the stone. Remove the pulp from the skins and cut into pieces. Mix with the grapefruit fillets, olives and the vinaigrette.
Peel garlic and chop finely. Peel the prawns, possibly down to the tail fins. Incise lengthways on the back, remove the dark intestine. Wash the prawns and pat dry.
Heat 1 tablespoon of oil in a pan. Fry the prawns in it, turning, for 3-4 minutes. Add the garlic and fry briefly. Season with salt and pepper.
Fold the lettuce into the avocado and grapefruit mix. Arrange everything and sprinkle with coarse pepper.