Cut onion in half and brown the cut surfaces in a pan. Add brisket, onion, bay leaves and peppercorns to a pot with enough water to cover meat. Bring to a boil and cook covered over medium heat for 2 hours.
Peel potatoes, celery and carrots. Cut potatoes into quarters, cut carrots into 1 cm (approximately 1/2 inch) thick slices and cut celery into 1 cm (approximately 1/2 inch) cubes.
Remove the meat from the broth and measure out 500 ml (approximately 2 cups) of broth. Heat butter and olive oil in a second pot. Sauté the chopped vegetables in it briefly, season with salt and pepper and add broth. Bring to a boil and cook covered over low heat for 15 minutes.
Trim fat from meat and cut into cubes. Add meat and leeks to the vegetables and cook for another 5 minutes. Chop the parsley finely and sprinkle over the stew. Combine horseradish with apple and serve with the stew.