Heat olive oil in a large skillet over high heat. Season the sirloin cubes with salt and pepper. Place them in a single layer and cook 3-4 minutes per side until nicely browned.
Remove the beef and set aside. Melt butter in the same pan, then add mushrooms and onions. Cook for 5-6 minutes until softened and golden, stirring occasionally.
Add garlic to the pan and cook for 30 seconds. Sprinkle in the flour and stir for 1 minute. Slowly whisk in beef broth, then add Worcestershire and soy sauce. Simmer for 2-3 minutes until thickened.
Return the seared beef to the pan and coat with the mushroom gravy. Cook for another 1-2 minutes until heated through. Sprinkle with parsley before serving.