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, for 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 of 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 for 1 more minute. Add back the beef and stir-fry for one more minute.
Reduce the 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 the dish with more black pepper, garnish with parsley, and serve.
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Notes
*It's easier to cut the beef into strips if you freeze it for about 20 minutes first. Or you could buy pre-sliced beef, marked as "stir fry beef."Removing the sour cream from the fridge at the start of cooking will help it warm up a bit, reducing the risk of it curdling when you add it to the hot dish. It also helps to reduce the heat and allow the skillet to cool off before mixing it in.