Put the beluga lentils in gently boiling water. Cook over medium heat for about 20 minutes until al dente. After 10 minutes add the red lentils and cook with them. Just before the end of the cooking time, salt and drain in a sieve, rinse and drain well.
Finely dice the onion. Clean and peel the carrots, celery and sweet potatoes and cut into approx. 1 cm cubes. Melt the rapeseed oil in a large saucepan. Sauté the onion in it over medium heat for 3-4 minutes.
Add the carrots, celery and sweet potatoes and cook for another 3-4 minutes. Season with salt and pepper. Add the curry powder, sauté briefly and fill up with the stock. Let simmer over a mild heat for about 25 minutes. Add the lentils and cook for a few minutes. If you want, you can lightly puree part of the stew for better binding.