Lightly coat the beef strips with salt, pepper and 1 teaspoon of olive oil.
Heat the remaining oil in a large frying pan over medium-high heat. Add in the beef strips and brown both sides, then transfer to a bowl and set aside.
Add the butter to remaining oil in the pan and cook until softened. Add in sliced mushrooms and onions and cook until the liquid has dried up.
Add in beef broth and cook until the sauce has thickened. Stir in sour cream, then return the browned meat strips to the pan. Cook for another 3 minutes until meat is heated through.
Add in chopped green onion and cook for 1 minute. Serve immediately.