Dice the onion and add to the vegetable stock together with the lentils, bring to a boil, cook for 50 minutes. After 40 minutes add the potatoes cut into cubes, after 45 minutes add the leek cut into rings. Season to taste with salt and pepper and pour into a baking dish.
Whisk the cream with the mustard, pour over it. Cook in a preheated oven at 250 ° C for 15 minutes.
Tip for non-vegetarians: Bratwurst goes well with it.