Season the sliced sirloin with salt and pepper. Heat the olive oil in a large skillet over high heat and sear the beef for 1-2 minutes until just browned. Transfer to a plate.
Reduce heat to medium and melt the butter in the same skillet. Add the onion and mushrooms, sautéing for 6-7 minutes until softened and golden.
Stir in the garlic and cook for 30 seconds until fragrant.
Pour in the beef broth, Dijon, and Worcestershire, scraping up the browned bits. Simmer for 3 minutes to reduce.
Lower the heat and stir in the heavy cream. Simmer gently for 4-5 minutes until thickened.
Remove from heat and stir in the sour cream until smooth. Return the beef and any juices to the skillet and warm through.
Sprinkle with fresh parsley and serve over cauliflower rice or zucchini noodles.