Soak the lentils overnight in plenty of cold water. Drain the next day.
Peel and finely dice the onion and garlic. Heat the oil. Sauté onion and garlic. Add 150 ml water and lentils, bring to the boil and simmer for about 5 minutes. Season to taste with vinegar, salt, pepper and sugar, let cool off.
Wash the cucumber, halve lengthwise, remove seeds and dice very finely. Mix with the lentils. Cut salmon into very thin slices. Arrange on plates. Season lightly with salt and pepper. Spread the lentils on top.
Cut the cress from the bed and sprinkle over it. Mix mayonnaise and wasabi, add to the mixture.