Place a large deep pan or dutch oven over medium-high heat. Add 2 Tbsp oil and once oil is very hot, add thinly sliced beef strips in a single layer, cooking 1 minute per side without stirring. Cook until just browned and no longer red. Sear beef in 2 batches so you don't overcrowd the pan. Remove beef to a plate and cover to keep warm.

Add 2 Tbsp butter, chopped onion and sliced mushrooms. Sautee 6-8 minutes or until liquid has evaporated and onions and mushrooms are soft and lightly browned.

Add 1 minced garlic clove and sautee 1 minute until fragrant. Add 1 Tbsp flour and sautee another minute stirring constantly.

Pour in 1 cups beef broth, scraping any bits from the bottom of the pan then add 3/4 cup whipping cream and simmer 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 Tbsp Worcestershire, 1/2 tsp dijon mustard, and season with 1/2 tsp salt and 1/4 tsp pepper, or season to taste and continue simmering until sauce is creamy. Add beef with any accumulated juices back to the pan and bring just to a simmer or until beef is heated through.