In the same skillet, add the sliced onion, minced garlic, and mushrooms. Sauté until the onions are soft and the mushrooms begin to brown, about 5 minutes.
Sprinkle the gluten-free flour over the vegetables and stir for about 1 minute to coat. Pour in the beef broth and add the Dijon mustard. Stir well and bring the mixture to a simmer.
Return the browned beef to the skillet. Allow the sauce to simmer for 10-15 minutes until it thickens slightly. Stir in the sour cream and adjust the seasoning with salt and pepper.