Cut the beef into thin strips against the grain and season with salt and pepper. Dredge with gluten-free flour and set aside.

Melt butter and olive oil in a skillet over medium heat. Add sliced mushrooms and diced onions, cooking until soft and golden brown.

Push the vegetables to the side and add the beef strips, searing for 2-3 minutes until browned.

Stir in minced garlic and cook for 1 minute. Pour in beef broth and Dijon mustard, mixing well and simmer for 5 minutes.

Remove from heat and stir in sour cream until smooth. Adjust seasoning to taste.

Serve over buttered egg noodles, mashed potatoes, or rice, garnished with parsley.