Rinse the cabbage and cut into large pieces. Finely dice onions. Rinse tomatoes, remove stem ends and blanch briefly in boiling water. Peel and cut in half. Rinse and peel the beets and potatoes. Bring the cabbage, tomatoes and onions to a boil in the broth and season with salt, pepper and bay leaf. Add meat, cover and cook 2 hours over low heat.
Take the meat out, cut into thin slices and keep warm. Remove vegetables (except potatoes) and the bay leaf with a slotted spoon. Puree broth with an immersion blender in the pot and season with salt, pepper, sugar and lemon juice.
Cut the cabbage into wide strips and cut beets into pieces. Add to soup and heat through. Rinse horseradish, peel and grate coarsely. Distribute meat in soup bowls. Pour the soup over and garnish with horseradish. Serve with fresh pita bread, if desired.