Heat the dripping in a large lidded pan over a medium-low heat and cook the onions slowly for 10 minutes until soft and golden. Do not rush this step, the onions are the flavour base of the whole dish.
Add the sausage chunks and fry for 5 minutes until browned all over.
Tip in the potatoes and pour over the stock. Add the Worcestershire sauce if using and season well with salt and pepper.
Cover and simmer gently for 25 minutes until the potatoes are completely soft and most of the liquid has been absorbed.
Take the lid off and beat the mixture hard with a wooden spoon until the potatoes break down and the whole thing turns rough and creamy, with flecks of onion and sausage running through it.
Taste and adjust the seasoning, then serve in bowls with pickled beetroot on the side, which is the traditional pairing.