Season beef strips with salt, pepper, and paprika, then set aside.
In a large skillet over medium-high heat, melt 2 tablespoons of butter and cook beef in batches for 2-3 minutes until browned but still slightly pink inside, then remove and set aside.
In the same skillet, melt remaining butter and sauté onions for 3-4 minutes until softened, then add mushrooms and cook for 5 minutes until golden.
Add garlic and cook for 1 minute until fragrant, then stir in beef broth, Worcestershire sauce, and Dijon mustard.
Bring to a simmer and cook for 3-4 minutes until slightly reduced, then reduce heat to low.
Stir in sour cream until well combined and smooth, then return beef and any accumulated juices to the skillet.
Heat through for 2-3 minutes without boiling, garnish with fresh parsley, and serve over cauliflower rice or shirataki noodles.