Slice the beef against the grain into thin strips, chop the onion, slice mushrooms, and mince garlic. Heat a large skillet over medium-high heat.

Pat the beef dry, season with salt and pepper. Add a tablespoon of oil to the hot pan and sear beef in batches for 1–2 minutes per side until browned but still slightly pink inside. Remove beef to a plate — don’t overcook.

Reduce heat to medium. Add a touch more oil if needed, then sauté the onion until translucent (3–4 minutes). Add mushrooms and cook until they release moisture and begin to brown (5–7 minutes). Stir in garlic for the last 30 seconds.

Sprinkle the flour over the vegetables and stir to coat. Cook 1 minute to remove raw flour taste.

Gradually pour in beef broth while scraping up brown bits. Add Worcestershire sauce. Simmer until the sauce thickens slightly, about 3–5 minutes.

Remove the pan from direct heat and stir in the sour cream until smooth. Return the beef (and any juices) to the pan and warm gently for 1–2 minutes. Avoid boiling after adding sour cream to prevent curdling.

Spoon over cooked egg noodles or rice and garnish with chopped parsley.