First Step: Prepare Ingredients Slice the beef sirloin thinly against the grain to ensure tenderness. Clean and thickly slice the mushrooms. Finely chop the onion and mince the garlic. Having these mise en place items ready will make the cooking process smooth and efficient.

Second Step: Brown the Beef Heat 2 tablespoons of oil in a large deep pan or Dutch oven over medium-high heat. Add the beef strips in batches, cooking 1 minute per side without stirring for a good sear. If using ground beef, crumble and brown until cooked through. Remove the beef and keep it warm.

Third Step: Sauté Vegetables In the same pan, add 2 tablespoons unsalted butter, chopped onion, and mushrooms. Sauté for 6 to 8 minutes until the liquid from the mushrooms evaporates and everything is golden and tender. Add minced garlic and cook for another minute until fragrant.

Fourth Step: Thicken the Sauce Sprinkle 1 tablespoon of flour over the mushroom mixture and stir to remove the raw flour taste, cooking for about 1 minute. Gradually pour in 1 cup beef broth, scraping browned bits from the pan to incorporate flavor. Stir in 3/4 cup heavy cream or evaporated milk and bring to a gentle simmer for 1 to 2 minutes until slightly thickened.

Fifth Step: Finish the Sauce Temper 1/4 cup sour cream by stirring in a few tablespoons of the hot sauce mixture to avoid curdling, then mix it back into the pan while stirring constantly. Add Worcestershire sauce, Dijon mustard, salt, and pepper. Allow the sauce to simmer gently until creamy and well combined.

Final Step: Combine and Serve Return the cooked beef with any juices to the pan and heat briefly. Cook egg noodles separately according to package instructions, or simmer noodles directly in the sauce with frequent stirring for a one-pot method. Garnish with chopped green onions or parsley. Serve immediately over noodles, mashed potatoes, rice, or kasha for a satisfying meal.