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, for about 2 minutes. Quickly remove the beef to a plate to prevent it from overcooking.
Add the onion and mushrooms to the saucepan with 1 more teaspoon of Diamond Crystal kosher salt (or ½ teaspoon of any other salt). Cook, stirring often, until tender, for 5-7 minutes. If the pan becomes too dry, add the juices 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.
Video
Notes
If using any salt other than Diamond Crystal Kosher Salt, including Morton kosher salt, use half the amount listed.
It's easier to cut the beef into strips if you freeze it for about 20 minutes first. You could also buy pre-sliced beef labeled 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 in the sour cream.
When you add the sour cream, the dish immediately takes on a grayish hue. It's still delicious, but if you'd like a deeper color, add a little Worcestershire sauce. However, don't add too much, or the dish will taste too vinegary. Start with one teaspoon and taste to see how it affects the flavor.
You can keep the leftovers in an airtight container in the fridge for up to three days. When reheating them, do so gently to avoid drying the meat and curdling the sour cream. I reheat the leftovers covered in the microwave at 50% power.