1. Preheat grill, place leeks in a bowl of ice-cold water with ice cubes added to it and leave until leeks curl slightly.
2. DRESSING: Blend garlic, mustard, salt, lemon juice and vinegar until smooth in a blender, or with a whisk. Whisk oil in gradually, until thickened. Whisk fresh cream in gradually but thoroughly.
3. Place fish on rack of a roasting pan or a baking sheet. Sprinkle with oil, salt, pepper and a little dill. Grill for 2 to 3 minutes a side. (It is important that the salmon trout retains its juiciness and texture, so grill it for the shortest possible time, under a fierce heat. Like most fish, salmon trout is better slightly undercooked; if it is overcooked, it will be dry.)
4. Using 2 forks, separate salmon trout into bite-sized pieces.
5. Spoon mangetout, patty pans, leeks, salad leaves and fish onto a large platter. Top with spinach. Sprinkle with cashew nuts, chopped egg and dill. Spoon dressing over and serve immediately.
Ideally, the salad leaves should be well chilled and crisp and the fish piping hot. This is one salad that has to be eaten immediately. If this is not possible however, cool the salmon trout to room temperature and pour the dressing over just before serving.