Heat the olive oil in a large pan over high heat. Season the steak with salt and pepper to taste.
Place the steak in the pan in a single layer; you may have to work in batches depending on the size of your pan. Cook for 3-4 minutes, stirring occasionally, until golden brown. Repeat with remaining meat if needed.
Add the butter and garlic to the pan; cook for 1-2 minutes, stirring to coat the meat in the sauce.
Sprinkle with parsley and serve.
If you don't have sirloin, you can also use tenderloin, rib eye, or strip steak.