Cut leeks in half lengthwise and rinse well. Tie each pair of leek halves together with cotton string. Lay leeks on potatoes. Cover and simmer 5 minutes. Discard any solidified fat from pot roast and sauce.
Scrape sauce from meat and add to broth.
With a slotted spoon, transfer equal portions of vegetables and meat to wide soup bowls and arrange attractively. Ladle hot broth equally into bowls. Accompany with mustard and horseradish to add to taste.