Cut the cooked beetroot into thin, bite-sized slices.
Mix the vinegar, oil, salt, pepper and marjoram separately in a cup with the dressing. The amount of spice is a matter of taste.
Finely chop the onion and fry in a little oil until the cubes are translucent. Then deglaze with the previously mixed salad dressing. Briefly bring to the boil. Then pour over the beetroot and mix. Possibly season to taste.