Mix the yogurt with the pressed, mashed or finely chopped garlic. Wash and grate the cucumber, add and season with salt and pepper, stir and place in the refrigerator.
Cook the lentils in salted water for about 15 minutes. Then reduce the heat so that the lentils only simmer gently. Add the rice. The water must cover the rice and the lentils. Season everything with salt and pepper and add a little cumin. Put the lid on the pot and simmer for about 15-20 minutes.
Meanwhile, cut the onions into strips and fry them brown in oil. When everything has risen well in the pot, mix in 3/4 of the onions.
Arrange the lentils on plates and garnish with the remaining onions. Serve with the yogurt.