Slice the beef sirloin thinly against the grain.

In a large skillet, heat the olive oil over medium-high heat.

Add the sliced beef to the skillet in a single layer. Sear for about 2-3 minutes until browned, then flip to cook the other side.

Once the beef is browned, remove it from the skillet and set aside.

In the same skillet, add the chopped onion and minced garlic. Sauté for about 2-3 minutes until the onion is translucent and fragrant.

Sprinkle the flour over the onions and garlic, stirring well.

Pour in the beef broth, Worcestershire sauce, and Dijon mustard. Stir together and bring to a simmer.

Reduce the heat to low and stir in the sour cream until well blended. Return the beef to the skillet to warm through.