Season the beef strips evenly with half of the salt and black pepper. Heat one tablespoon of oil in a large skillet over high heat until shimmering.

Sear the beef strips in batches for about 60 to 90 seconds until browned on the outside but still pink in the center. Immediately transfer the cooked beef to a plate and keep warm.

Lower heat to medium-high. Melt the butter with the remaining tablespoon of oil in the same skillet. Add the sliced mushrooms and cook without moving for 2 to 3 minutes to develop color, then stir and cook until tender and golden brown.

Add the diced onion to the skillet and cook for 3 to 4 minutes until soft and translucent. Stir in the minced garlic and cook for 30 seconds until fragrant.

Dust the all-purpose flour over the vegetables and stir continuously for 1 minute to coat and cook out the raw flour flavor.

Gradually pour in the beef broth while stirring and scraping up any browned bits stuck to the bottom of the pan. Whisk in the Worcestershire sauce and Dijon mustard. Bring to a gentle boil, then reduce heat and simmer until the gravy thickens slightly.

Turn the heat down to low. Stir in the room-temperature sour cream until the sauce is completely smooth and silky. Return the beef and any resting juices back to the skillet. Heat gently for 1 minute to warm through without bringing to a boil.

Taste and adjust seasoning with remaining salt and black pepper. Garnish with chopped fresh parsley and serve immediately over buttered egg noodles or mashed potatoes.