For the trout, place the trout fillets on a chopping board, lay a small handful of watercress at one end of each fillet, season well, roll up and secure with toothpicks.
Place the trout fillets in a steamer, set over a pan of boiling water and steam for six minutes, or until cooked through.
For the sauce vierge, place the tomato, lemon juice, olive oil, onion, coriander and shrimps into a small saucepan and season, to taste, with salt and freshly ground black pepper.
Gently warm through, then stir in the basil.
Cook the potatoes in a large pan of boiling salted water until tender. Drain well.
To serve, place the steamed trout in the middle of a serving plate, dot the potatoes round the plate and pour over the sauce vierge.