Rinse the lentils in a sieve and cook in boiling water for about 30 minutes. In the meantime, sort out the rocket, wash and drain. Cut the beetroot into slices (Attention, strong colouring - wear disposable gloves!).
Onions peel, halve and in fine strips cut. Wash the meat, dab dry and cut into cubes.
Heat the oil in a large frying pan. Brown the meat in it all around for about 5 minutes. Season with salt and pepper. Take out. Fry the onions in the frying fat. Add beetroot, 5 tablespoons water and vinegar, bring to the boil.
Drain the lentils and put them in the pan with the meat, warm them up. Mix in the rocket well. Season to taste with salt, pepper and 1 pinch of sugar.