Rinse the lenses in a sieve under running water and drain well.
Peel onion and chop finely. Wash the leeks, cut them in halves and cut them transversely into strips of 1 cm. For the marinade mix vinegar, salt, pepper and marjoram. Stir in 4 tbsp. oil. In the remaining oil sautée leeks and onion, fold in lentils. Pour mixture into a bowl, mix well with the marinade and marinade for about 10 minutes. Divide the trout fillets and mix them with the salad.