Heat the oil and one tablespoon of the butter in a large frying pan over medium heat.
Add the onions and a pinch of salt.
Sauté until the onions begin to soften.
Add the remaining tablespoon of butter to the pan.
Add the mushrooms and cook for about five minutes, stirring frequently.
Remove the onions and mushrooms from the pan. Cover and keep warm.
Add the remaining tablespoon of olive oil to the pan.
When it is warm, add the slices of steak and stir fry until nearly cooked.
Add half a cup of the sour cream or crème fraîche, the mustard, pepper and paprika.
Return the onions and mushrooms to the pan, adding a bit more sour cream if necessary. Stir and warm through.
When everything is warmed through, add the nutmeg.
Serve over the rice.