Measure out the sour cream and remove it from the fridge, to allow it to get to room temperature.
Season the beef with 1 teaspoon kosher salt and ¼ teaspoon black pepper.
Heat the oil in a large saucepan over medium-high heat, about 2 minutes. Add the beef. Cook, stirring, just until no longer raw, about 2 minutes. Quickly remove the beef to a plate to prevent it from over-cooking.
Add the onion and the mushrooms to the saucepan, with 1 more teaspoon kosher salt. Cook, stirring often, until tender, 5-7 minutes, also adding the juices that have accumulated from the resting meat.
Add the garlic and cook 1 more minute. Add back the beef and stir-fry one more minute.
Reduce heat to low and allow the skillet a few seconds to cool down a bit. Add the sour cream, mixing it well with a wooden spoon into the beef mixture. Don't allow the sour cream to simmer or boil, to avoid curdling. Season with more black pepper and serve.
*It's easier to cut beef into strips if you freeze it for about 20 minutes first. Or you could buy pre sliced beef, marked as "stir fry beef."