Rassolnik (Russian: рассольник) is a traditional Russian soup made from pickled cucumbers, pearl barley, and pork or beef kidneys. A vegetarian variant of rassolnik also exists and usually made during a lent. The dish is known to have existed as far back as the 15th century, when it was called kalya.
The key part of rassolnik is the rassol, a liquid based on the juice of pickled cucumbers with some other additions. It is also famous as a hangover treatment.