Sprinkle beef with salt and pepper. Heat the cooking fat in a saucepan and sear the beef on all sides; you may have to do this in batches to avoid overcrowding the saucepan.
Add the pancetta, onions, carrots, celeriac and leek then fry for a few minutes. Season lightly with salt and pepper then dust with the flour and add the tomato paste. Stir well.
Pour in half of the stock. Cover and cook for 1 1/2 hours over a low heat.
Peel and dice the potatoes the same size as the diced beef then add to the beef with the rest of the stock. Cook for a further 30 minutes and sprinkle the finished goulash with parsley.