Slice the endives as finely as possible lengthways. Mix the sliced endive with the extra virgin olive oil, the zest and segments of the grapefruit and half of the juice. Add half of the sliced chives, and the baby plum tomatoes. Season well with salt and cayenne, set aside.
Next, heat a large non-stick frying pan with the vegetable oil. Add the tiger prawns and sauté for a minute or so each side. Add the second half of the grapefruit juice, the remaining chives and the chopped chilli. Toss until the prawns are well coated.
To finish, toss the picked watercress with the fennel salad and spread it all onto a large plate, top with the prawns and serve.