Prep the beef: Pat the sliced beef dry with paper towels. Season generously with salt and pepper. Dry beef browns better and helps build flavor.

Sear in batches: Heat 1 tablespoon oil in a large skillet over medium-high heat. Add half the beef in a single layer. Sear 1–2 minutes per side until browned but still pink inside. Transfer to a plate and repeat with remaining oil and beef. Don’t overcook—it will finish in the sauce.

Sauté onions and mushrooms: Reduce heat to medium. Add butter to the pan. Stir in onions with a pinch of salt and cook 3–4 minutes until softened. Add mushrooms and cook 5–7 minutes, stirring occasionally, until they release moisture and lightly brown. Add garlic and cook 30 seconds until fragrant.

Build the sauce base: Sprinkle flour over the vegetables and stir for 30–60 seconds to coat. Pour in the wine (or broth), scraping up browned bits. Let it simmer 1–2 minutes to reduce slightly.

Add broth and seasonings: Stir in 1 cup broth, Dijon mustard, and Worcestershire sauce. Simmer 3–4 minutes until slightly thickened and glossy.

Finish with sour cream: Reduce heat to low. Whisk a few spoonfuls of the hot sauce into the sour cream to temper it, then stir the sour cream into the pan. Add lemon juice if using. Taste and adjust salt and pepper.

Return the beef: Add the seared beef and any juices back to the skillet. Warm gently 1–2 minutes, just until heated through. Avoid boiling to keep the sauce silky and the beef tender.

Serve: Spoon over egg noodles, rice, or mashed potatoes. Garnish with chopped parsley and a few cracks of black pepper.