Heat 2 tablespoons of olive oil in a large deep pan over medium-high heat.

Add the thinly sliced top sirloin steak to the hot pan in a single layer. Sear for about 1 minute on each side until browned.

In the same pan, reduce the heat to medium and add 2 tablespoons of unsalted butter, then add 1 finely chopped medium onion and 8 ounces of thickly sliced brown mushrooms.

Sauté for 6-8 minutes until the onions are translucent and mushrooms soften.

Stir in 1 minced garlic clove and sauté for 1 more minute until fragrant.

Sprinkle 2 tablespoons of all-purpose flour over the mixture and stir well, cooking for 1 additional minute.

Gradually pour in 2 cups of beef broth, scraping the bottom to release brown bits, then bring to a gentle boil and simmer for 3-4 minutes until thickened.

Lower the heat and stir in 1 cup of heavy whipping cream, cooking for 3-5 minutes until thickened.

Temper 1 cup of sour cream with a spoonful of the heated sauce before adding it along with 1 tablespoon of Worcestershire sauce and 1 teaspoon of Dijon mustard. Season with salt and black pepper to taste.

Return the seared beef to the pan and mix well, heating through for 2-3 minutes.

Serve over cooked egg noodles or rice and garnish with chopped green onions or parsley.