1️⃣ Season & Sear the Beef
Season beef with salt and pepper.Heat olive oil in a large skillet over medium-high heat.Sear beef quickly (1–2 minutes per side). Remove and set aside.2️⃣ Cook the Vegetables
In the same skillet, melt butter.Add onions and cook until soft and lightly golden (5–7 minutes).Add mushrooms and cook until browned and moisture evaporates.3️⃣ Build the Sauce
Add garlic and cook 30 seconds.Stir in Dijon mustard and Worcestershire sauce.Sprinkle flour (if using) and cook 1 minute. Sloly add beef broth, stirring until smooth. Simmer 3–4 minutes.4️⃣ Finish with Cream
Lower heat. Stir in sour cream gently until smooth.Do not boil.5️⃣ Combine & Serve
Return beef to the skillet and warm through (2–3 minutes).Season to taste and garnish with fresh parsley.