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, usually made during Lent. The dish is known to have existed as far back as the 15th century, when it was called kalya. It is also common in Poland, where it is known as zupa ogórkowa (literally cucumber soup).
The key part of rassolnik is the rassol, a liquid based on the juice of pickled cucumbers with various other seasonings. It is a favourite hangover treatment.