Season the beef slices with salt and pepper, then sprinkle with flour, tossing to coat evenly. Shake off any excess flour.

In a large skillet, heat the vegetable oil over medium-high heat. Add the beef slices in batches, browning them on both sides. Once browned, remove from the skillet and set aside.

In the same skillet, melt butter and reduce heat to medium. Add the onions and garlic, cooking until the onions are tender and translucent, about 3-4 minutes.

Add the sliced mushrooms to the skillet. Stir frequently and cook until golden brown and any liquid has evaporated, around 5 minutes.

Pour in the beef broth, Worcestershire sauce, and Dijon mustard. Stir well, scraping any flavorful bits from the bottom of the skillet.

Place the beef back into the skillet, reduce the heat to low, and let it simmer for 5-7 minutes until the beef is tender and heated through.

Stir in the sour cream gently until fully incorporated, heating through but ensuring it doesn’t boil to avoid curdling.

Enjoy this Stroganoff over cooked egg noodles or rice, garnished with fresh dill or parsley if desired.