Sprinkle beef with salt and pepper. Heat the cooking fat in a pan and sear the beef on all sides (you may have to do this in batches to avoid overcrowding the pan). Add the bacon, onions, carrots, celeriac and leek and fry for a few minutes. Season lightly with salt and pepper, dust with the flour and add the tomato puree. Stir well and pour in half of the stock. Cover and stew for 1 1/2 hours over a low heat.
Meanwhile, peel and dice the potatoes - the same size as the diced beef - and add to the beef with the rest of the stock. Cook for a further 30 minutes and sprinkle the finished goulash with finely chopped parsley.