Cut the steak into thin strips about 4-6 cms long. Using a large frypan, fry the onions until soft, transfer to another dish. Fry the mushrooms lightly and add to the onions. Add a little extra butter to the pan if needed and brown the steak on high heat a batch at a time, liberally sprinkling with nutmeg and pepper plus a little salt, until just cooked (i.e. no longer pink on the outside but still tender) - only needs a minute or so. (Don't overcook, especially if using rump, or it will go tough).

Return mushrooms and onions to the pan, stir in sour cream and tomato paste, and reheat without boiling. Adjust seasoning as needed - I actually like quite a bit of nutmeg. Serve with rice.