In a large skillet, cook the ground beef over medium-high heat until browned. Drain off any excess grease.
Add the chopped onion, minced garlic, and sliced mushrooms to the skillet. Cook until the vegetables are softened, about 5-7 minutes.
Sprinkle the flour over the beef and vegetable mixture and stir to coat. Cook for 1 minute.
Gradually stir in the beef broth. Bring the mixture to a simmer, scraping up any browned bits from the bottom of the skillet.
Reduce the heat to low and stir in the sour cream, butter, salt, and pepper. Cook until the sauce has thickened and is heated through, but do not boil.
Serve hot over noodles, rice, or mashed potatoes.
Notes
This Stroganoff is best served immediately. For a richer flavor, you can add a splash of Worcestershire sauce.