Preheat oven to 350°F.

Combine all stock ingredients in a deep baking pan or oven-safe pot and add water to cover. Cover and bake for three hours. 

Let cool and refrigerate overnight. Remove congealed layer of fat and discard.

Remove cooked flanken meat from pan. Shred meat into small pieces. Discard any fatty pieces.

Remove bones and place in a mesh bag. Place the mesh bag, meat pieces, and pan juices in a large pot. Add about six cups of water and bring to a simmer.

Heat oil in a saute pan over medium heat. Add onion and saute about five minutes. Add carrots and beets. Saute for 20–25 minutes, stirring continuously, until softened.

Add vinegar (or lemon juice). Raise heat and bring to a boil. Add potato and garlic, and let simmer for 20 minutes to half an hour, until potatoes are fork tender. Season with salt and pepper to taste. Remove the mesh bag and discard bones.