Clean, peel and finely dice the carrots and celery. Then sweat the vegetables with the butter and lentils. Deglaze with 150 ml of white wine, pour in the stock and simmer for about 20-30 minutes with the lid closed.
Chop the dill and mix with the crème fraîche, the two types of mustard and lemon juice and season with salt, pepper and sugar. Cut the leek into fine rings.
Mix the leek, mustard sauce and the lentils and reduce again for a few minutes. Season to taste with salt and pepper again.