In a large skillet over medium heat, melt the butter until it starts to foam.

Add the chopped onion and minced garlic to the skillet, sautéing until the onion turns translucent, about 3-4 minutes.

Toss in the sliced beef sirloin and mushrooms, cooking until the beef is browned, approximately 5-7 minutes.

Sprinkle the flour over the beef mixture, stirring to combine and cook for about 1 minute.

Gradually pour in the beef broth while stirring continuously, ensuring no lumps form. Bring the mixture to a gentle simmer.

Let the sauce simmer for about 5 minutes, or until it thickens to your desired consistency.

Remove the skillet from the heat and gently fold in the sour cream. Season with salt and pepper to taste.

Ladle the Beef Stroganoff over cooked egg noodles or rice, and finish with a sprinkle of fresh parsley.