Onions peel, halve and cut into strips. Drain the lentils in a sieve. Beetroot also drain and cut into slices. Wash the meat, dab dry and cut into large pieces.
Season meat with salt and pepper.
Heat the oil in a large frying pan. Fry the onions for 3-4 minutes, turning them over. Add meat and fry while turning until the meat has a little colour. Add lentils, beetroot, vinegar and 50 ml water.
Cover and cook for 5-7 minutes. Season to taste with salt and pepper.
Wash the rocket and shake dry thoroughly. Add the rocket to the pan and mix well. Serve immediately.