We picked up this recipe from the cookbook "The Handbook of the Russian Experienced Household" by Ekaterina Avdeeva, a writer, ethnographer, and a traveler who lived in the 19th century.
Her book included many great recipes collected from various regions of Russia, including Siberia. In the middle of the 19th century families were often very large, so the amount of food per dish usually looks terribly large.
Ekaterina writes that oat soup was extremely popular in St. Petersburg. The classic recipe is non-Lenten, as there is butter, but it is also possible to make a lean version, replacing the butter with vegetable oil. From the Russian name it is clear that this soup came from German cuisine, specificallу from Prussia.
Wash the oat groats thoroughly, pour with water (2 bottles per cup), boil and sieve. Add prunes, a small piece of butter (or vegetable oil) butter, and let it boil to make the prunes soft.
Since we recommend this soup for the Lenten table, butter should be replaced with vegetable oil.