First off on high heat, heat some of the olive oil in a cast iron pot or wok – these types of pans just keeps the heat very well, just before smoking.
Add ¼ of the beef strips to the pan and sauté (or flash fry as I like to call it) the beef strips until brown. Remove from pan and repeat with the rest of the meat by only adding ¼ at a time. You really don’t want to add all the meat to the pot otherwise you will end up cooking it rather than just sautéing. Season with salt and black pepper.
In the same pan/pot, add a little more oil and the onions. Add the water to deglaze the pan, add the garlic and mushrooms and leave to cook until soft and translucent.
Add the sour cream, and mix thoroughly until incorporated. If the sauce is still a bit thin, leave to cook for another 5 minutes to reduce.
Add beef strips, stir, and season with salt and pepper if needed and serve on rice or pasta.