Shred the cabbage into thin strips and blanch in boiling in boiling water for a minute or so, then drain.
Alternatively pour hot water over any leftover cooked vegetables, then drain (you can use leftover steamed cabbage too).
Mash the potatoes with a fork or mixer. Mix in the butter and the cabbage strips, then season with salt and pepper.
In a pan heat some oil and cook the onion until soft, then add the cabbage and potatoes mixture, and press down. Cook for 3 minutes until brown, then turn over and cook the other side until brown.
Alternatively make the mixture into small patties and fry them individually (these look more attractive & appetizing).
Serve with any leftover meat and warmed up gravy.