Boil the beets, peel, slice or grate them into thin strips.
Cut the onion into thin rings.
Scald the onion with boiling water.
Cut mushrooms, I took pickled mushrooms. If you take fresh or frozen, then boil them accordingly and also cut them.
Mix everything in a salad bowl. Add pepper if desired. Make a dressing from vinegar (you can take lemon juice, citric acid), water, salt, sugar and vegetable oil. Pour the salad, let it infuse, put it on a hill, decorate with a salad leaf, any greens (I have coriander), olives.
To clarify, I took half a teaspoon of acetic acid (70%) and 50 ml of water for refueling.