Peel the onion and cut into fine cubes. Heat 1/2 teaspoon of oil in a pot and fry half of the onion in it until transparent. Add the lentils and fry briefly. Add 100 ml water, season with marjoram.
Bring to the boil and simmer for 10-15 minutes. In the meantime clean, wash and cut the peppers into small cubes. Clean and wash the celery and cut it into thin rings. Wash the salad and dab dry. Halve the salad leaves.
Add the celery to the lentils 3 minutes before the end of the cooking time. Drain the lentils in a sieve. Wash the parsley, dab dry. Pluck the leaves from the stalks and cut them finely. Mix the remaining onion, half of the parsley, mustard and vinegar.
Season with salt, pepper and sugar. Add 1 teaspoon of oil. Mix lentils with vinaigrette. Pluck the trout filet into pieces. Add paprika and trout fillet to the salad and let it steep for about 5 minutes.
Arrange lentil salad on salad leaves, garnish with remaining parsley.