Jewish Beet Soup

From 1918:

To make Jewish beet Soup, one has to have what they call sour salt. It can be purchased at a Jewish store.
Here’s the recipe:
Ten medium sized beets (not too large), 1 thimble sour salt, 1 ½ pounds fat breast beef. Chop beets small, add sour salt and boil with beef for four or five hours. Season with salt, pepper and a quarter cup of sugar. When removing from stove, add beaten egg. Some cooks chop an onion and add it at same time as the beef.