Freeze the sirloin for 20 minutes to make it easy to cut. Then cut into thin slices.

Heat 1 tbsp of olive oil in a large skillet over high heat. Season beef with salt and pepper, then sear quickly in batches until browned (30-60 seconds per side). Remove from the pan and set aside.

Reduce heat to medium. In the same pan, add the remaining olive oil and chopped onion and cook for 2–3 minutes until softened.

Add garlic and mushrooms, cooking for another 5–7 minutes until mushrooms release their moisture and begin to brown.

Sprinkle gluten-free flour over the vegetables. Stir and cook for 1 minute.

Slowly pour in the beef broth while stirring to avoid lumps. Add Dijon mustard and Worcestershire sauce and bring to a simmer.

Cook until slightly thickened, about 5 minutes.

Reduce heat to low. Stir in sour cream until smooth and warmed through (do not boil).