Add 1 Tbsp olive oil in a large skillet. Once oil is very hot, add beef and cook until until just browned. Remove beef from the pan and set aside.
Add chopped onion and sliced mushrooms to the pan. Saute 6-8 minutes or until liquid has evaporated and onions and mushrooms are soft and lightly browned.
Add 1 minced garlic clove and saute 1 minute until fragrant. Add 2 Tbsp flour and saute another minute stirring constantly.
Pour in 1 cups beef broth, scraping any bits from the bottom of the pan then add 1 cup 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 1/2 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.