Peel potatoes, cutting any large ones in half so that they are all the same size. Submerge potatoes in boiling, salted water and add the stock cube.
Put the kale on top of the potatoes and the rookworst on top of the kale.
Cover with a lid, bring to the boil then simmer for 20 minutes until the potatoes are cooked.
While the potatoes and kale are cooking, heat bacon with a spoonful of lard or butter in a heavy saucepan. Add the onions and a pinch of salt. Braise for 15 minutes so that the onions are caramelised but the bacon is not burnt. When the potatoes are cooked, remove rookworst and drain, retaining liquid. Mash potatoes and kale without turning into a purée; add cooking liquid to obtain the right consistency.
Mix in bacon and onions with lots of pepper and a dollop of butter or lard. Add the vinegar and season to taste. Serve piping hot with the rookworst on top, either whole or in thick slices. Gravy is optional.
1 meat stock cube
600g kale, chopped 1 rookworst sausage (if you can’t find rookworst use Frankfurters, although they are really not the same) 100g bacon, chopped
50g lard or butter
2 onions, chopped
2 tbsp white wine vinegar
Salt and black pepper