Make the marinade by mixing the olive oil, soy sauce, lemon juice, red wine vinegar, Worcestershire sauce, parsley, garlic, black pepper, and dried mustard in a large Ziplock bag. Add flank steak and marinate for 6-8 hours (or overnight).
Remove the steak from the fridge 20 minutes before grilling.
Heat a grill pan over medium-high heat. Coat the grill pan with olive oil cooking spray.
Grill the steak for 4-5 minutes on each side for medium-rare, or until your desired degree of doneness.
Let the meat rest for at least 5 minutes before slicing the flank steak against the grain into thin slices.
Boil the remaining marinade in a small saucepan for 3-5 minutes, for extra sauce, if desired. Drizzle the sauce over the sliced steak.