To make the marinade, combine the oil, vinegar, garlic, mustard, oregano, parsley, rosemary, Worcestershire and some salt and pepper in a food processor and pulse until the garlic and herbs are incorporated in the oil.
Place the steaks in a ziptop bag, pour the marinade over the steaks and marinate at least 30 minutes and up to overnight.
Before grilling, remove the steaks from the bag and pour the remaining marinade into a small pan. Bring the marinade to a boil, lower to a simmer and cook for 2 to 3 minutes. Use this sauce to drizzle on the steaks as soon as you remove them from the grill.
Prepare a gas or charcoal grill for high heat. When up to temperature, grease the grates with a little oil, then place the steaks over the hottest part of the grill. Depending on the thickness of the steaks and the temperature of the grill, the steaks should grill about 3 minutes per side for medium rare (flank steak is best served medium rare). Remove the steaks from the grill, drizzle with the sauce and let them rest for 5 to 7 minutes.