Place the sliced beef in a bowl and season with half of the salt and pepper. Sprinkle the flour over the beef and toss gently until each piece is lightly coated. This helps the beef brown and later thickens the sauce.
Heat a large, heavy skillet over medium-high heat. Add the olive oil and 1 tablespoon of butter. Once hot, add the beef in a single layer. Cook quickly for about 1 minute per side, just until browned. Do not overcrowd the pan. Remove the beef to a plate and set aside.
Reduce the heat to medium. Add the remaining butter to the same pan. Add the sliced onions and cook for 3–4 minutes until soft and lightly golden, scraping up any browned bits from the pan.
Add the mushrooms and cook for 5–6 minutes until they release their moisture and turn golden brown. This step builds deep flavor, so let them cook undisturbed for short intervals.
Stir in the minced garlic and cook for 30 seconds until fragrant. Add the Worcestershire sauce and Dijon mustard, stirring to combine.
Slowly pour in the beef stock, stirring constantly to create a smooth sauce. Bring the mixture to a gentle simmer and cook for 5–7 minutes until slightly thickened.
Lower the heat to medium-low. Return the beef and any resting juices to the pan. Stir gently and simmer for 2–3 minutes until the beef is just cooked through and tender.
Remove the pan from heat. Stir in the sour cream and lemon juice until fully incorporated. Taste and adjust seasoning with the remaining salt and pepper as needed. Do not boil after adding the sour cream to keep the sauce smooth.
Sprinkle with chopped parsley just before serving.
