Sour Salt, btw, is food grade citric acid. It used to come in little chunks, pea size and smaller, in a little bottle, but these days, it comes more like a course salt to fine salt.
This has been made in our family for generations. I make it, my mother made it, my grandmother made it, and my great-grandmother made it. I have no idea how far back it went before that.
When my great-grandmother made it, my grandmother's uncle (my grandmother, 2 sisters, and their mother, lived with 'Uncle' after the father died young.) would take a small bowl and put some red pepper flakes in it, along with a little of the soup broth, and put it on the table. Then, anyone who wanted a bite of heat could put a little of the spicy liquid into their own bowl.
The way that we eat this is to serve it piping hot, and as you eat it, you take the pieces of meat out of your bowl, and put them, one at a time, on a piece of your bread - rye is traditional, (I serve it on a salad/bread plate next to each bowl.) but any good bread works, generally spread with good, grainy mustard, or horseradish, but that's up to you. You do this throughout the meal, eating first a piece of meat folded in some bread, then eat some of the soup itself, etc.
Meat, cut into bite sized pieces, kind of like stew.
Can or Jar of Sauerkraut, drained and rinsed.
Can of Tomatoes, large, cut up. Use liquid.
Onion, whole was traditional, but myself, mom, and grandmother always cut it into quarter ringlets.
Water - There's no set amount, 2-4 cups maybe? You can add as you need to, but you don't want to water it down too much.
Salt & Pepper to taste.
Sugar - About a teaspoons to taste. This is not a sweet soup, but it does need balance with the 2 main ingredients. Start with less, and add a little at a time.
Sour Salt - Sour salt used to come in rocks. I still have trouble converting grains to the rocks that we all used when I was growing up. Don't use too much. Start w/ 1/2 pea size. Test. If not enough, put in another 1/2 pea size. It will probably not take more than that, but use your taste. It's not a sour tasting soup, but shouldn't be bland, either. Lemon juice does not make a good substitute for sour salt.
Bread (rye, if possible, but any good bread.)
Horseradish or Good Mustard, such as whole grain, or brown, but whatever works for you.
Put everything in the pot and cook until done. The onions are not sauteed first.
You want the ingredients to really cook down together, and be well done. My mother cooked it for a few hours. It doesn't require quite that long, but it does need time to really melt into a good soup. I cook this soup longer than most soups that I make.
Before serving, taste and adjust seasonings.
Serve with bread, and condiments.