Peel the onions and cut into fine rings and fry in the oil until crisp, then drain in a colander. Peel and finely dice the shallots. Remove the seeds from the chili pepper and dice finely. Sauté chili, lentils, shallots and ground coriander in 30 g butter. Add 400 ml stock and swell at low temperature for 20-25 min. Stir a few times.
Chop the coriander greens, put a few leaves aside. Stir the sour cream or crème fraiche into the lentils and blend finely with a hand blender. Season with salt and pepper. Finally, stir the chopped coriander greens into the lentils. Cover and keep warm.
Clean the leeks and cut them into 4 cm long diagonal pieces. Heat the remaining butter in a frying pan. Briefly sweat the mustard seeds and curry powder in it. Add the leek and turn it to the other side in the fat. Add the rest of the stock and cook for 7-8 minutes with the lid on. Season with pepper.
Arrange the lentil puree with the onion rings and leek pieces on plates and garnish with coriander leaves.