Heat a large deep pan or Dutch oven over medium-high heat. Add 2 tablespoons of oil and once the oil is very hot, add the thinly sliced beef strips in a single layer. Cook for 1 minute per side without stirring, until just browned and no longer red. Cook the beef in 2 batches to avoid overcrowding the pan. Remove the beef to a plate and cover to keep warm.

Add 2 tablespoons of butter, chopped onion, and sliced mushrooms to the pan. Sauté for 6-8 minutes or until the liquid has evaporated and the onions and mushrooms are soft and lightly browned.

Add 1 minced garlic clove and sauté for 1 minute until fragrant. Then add 1 tablespoon of flour and sauté for another minute, stirring constantly.

Pour in 1 cup of beef broth, scraping any bits from the bottom of the pan, then add 3/4 cup of whipping cream and simmer for another 1 to 2 minutes or until slightly thickened.

Stir a few tablespoons of the sauce into 1/4 cup of sour cream to temper it so the sour cream doesn’t curdle, then add it to the pan while stirring constantly.

Stir in 1 tablespoon of Worcestershire sauce, 1/2 teaspoon of Dijon mustard, and season with 1/2 teaspoon of salt and 1/4 teaspoon of pepper, or season to taste. Continue simmering until the sauce is creamy.

Add the beef with any accumulated juices back to the pan and bring just to a simmer or until the beef is heated through.