Rinse and drain the lenses. Peel and finely chop the onion. Heat 1 tablespoon of oil in a saucepan. Fry the onion in it. Add the lentils and 1/2 litre water, bring to the boil, cover and cook for 40-45 minutes.
Let the fish thaw for 20-30 minutes. Clean, wash and cut the leek into rings. Dab fish dry, cut into large pieces and season with salt and pepper. Beat the egg and 2 tablespoons of cold water, turn the fish in it.
Heat 4 tablespoons of oil in a frying pan. Fry the fish for 4-6 minutes while turning. Add the leek to the lentils about 5 minutes before the end of the cooking time. Add cream and season with salt. Stir in mustard and vinegar.
Arrange lentils and fish on plates.