Heat the oil in a large frying pan or skillet, when the oil is hot add the butter and melt the butter in the oil.
Add the onion and sauté the onions till they are translucent.
Add the mushrooms and cook for a further 3-4 minutes, stirring often, until lightly browned.
Pour out the onions and mushrooms into a bowl and return the pan to the stove with the heat on high.
Dust the beef strips with some flour.
Drizzle the strips of beef with some olive oil and sprinkle on the salt and pepper.
Place the steak in the pan in a single layer and brown for about a minute on each.
Return the onions and mushrooms to the pan and turn the pan down to medium heat.
Pour the cream followed by the sour cream in the pan and in stir well.
Reduce the heat and allow the sauce to simmer and cook for 10-15 minutes.
