Heat 1 tablespoon of the olive oil in a large (preferably non-stick) frying pan over medium heat.
Add the onions and sauté, stirring almost constantly.
Add a generous pinch of salt to prevent the onions over browning.
Add half of the remaining tablespoon of oil to the pan.
Add the mushrooms and sauté for a further three to four minutes, stirring frequently.
Add the last of the oil to the pan and toss in the thinly sliced steak.
Sauté the steak for a minute or two. It should still be slightly pink.
Stir in the beef stock along with the mustard, pepper and paprika.
Stir in ¼ cup of the light sour cream or crème fraîche.
Warm everything through and when the steak is cooked (just another minute or so), stir in the nutmeg and then check the stroganoff for texture and flavour.
Add more of the light sour cream or crème fraîche if required and season to taste with more salt and pepper if necessary.
Serve the stroganoff over the hot rice on warm plates.