Start by gathering all your ingredients. Slice the beef thinly against the grain, chop the onions, minced garlic, and slice the mushrooms. Having everything ready will streamline the cooking process.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the onions and sauté for about 3-4 minutes, or until they become translucent. Add in the garlic and mushrooms, cooking for an additional 5 minutes until the mushrooms become tender and release their moisture.

Push the vegetables to one side of the skillet, and increase the heat to medium-high. Add the sliced beef to the empty side of the pan, seasoning it with salt and pepper. Cook for 2-3 minutes, stirring occasionally, until the beef is browned but not fully cooked through.

Sprinkle the flour over the beef and vegetables, stirring to combine. Pour in the beef broth while stirring to prevent lumps from forming. Add the Worcestershire sauce and Dijon mustard. Bring the mixture to a gentle simmer and cook for about 5 minutes, allowing the sauce to thicken.

Reduce the heat to low and stir in the sour cream until well incorporated. Allow it to heat through for about 2-3 minutes, but do not let it boil. You want a creamy, luscious sauce that clings to the beef and veggies.

Remove the skillet from heat and garnish with fresh parsley. Serve immediately over egg noodles, rice, or creamy mashed potatoes.