Trim the kidneys by removing the gristle and cut the remainder into bite size slices and season.
Heat the oil in a wok and stir fry the kidneys for about 2 minutes. Add the butter and continue to stir fry for another 2 minutes. Remove the kidney with a slotted spoon and keep aside.
Stir fry the onions/garlic in the same oil until soft (around 4 minutes). Add the vinegar and stir fry for a further 2 minutes. Prepare the mustard, add to the wok and then stir in the yoghurt a little at a time. Add the lime juice and cook for a further minute and then return the kidneys to the wok. Stir well, cover and simmer for 5 minutes.