Freeze the sirloin for 20 minutes to make slicing easier. Cut into thin strips, then season lightly with salt and pepper.
Heat 1 tablespoon of olive oil in a large skillet over high heat. Sear the beef in batches, cooking for 30-60 seconds per side until browned. Remove from pan and set aside.
Reduce heat to medium and add the remaining tablespoon of olive oil. Add onions and sauté for 2-3 minutes until softened and fragrant.
Stir in garlic and mushrooms, cooking for another 5-7 minutes until mushrooms release their moisture and begin to brown.
Sprinkle gluten-free all-purpose flour over the vegetables and stir well to coat evenly. Cook for 1 minute to eliminate any raw flour taste.
Gradually pour in the beef broth, stirring constantly to avoid lumps. Add Dijon mustard and Worcestershire sauce, then bring to a gentle simmer. Cook for 5 minutes until slightly thickened.
Reduce heat to low and stir in sour cream until fully incorporated and smooth. Avoid boiling to prevent curdling.
Return the seared beef to the skillet and simmer for 2-3 minutes until heated through. Adjust seasoning with salt and pepper as needed.
Serve the stroganoff over gluten-free pasta, mashed potatoes, or rice. Garnish with parsley, if desired, and enjoy!