Clark Frasier's mom used to serve this sauce with grilled salmon, and it's a favorite standby condiment at Arrows restaurant. It is terrific with salmon, but also particularly tasty with flank steak, a flavorful cut of meat. By marinating the steak in the sauce you're adding even more flavor to this delicious cut of meat.
In a large bowl combine the red wine vinegar, balsamic vinegar, Worcestershire sauce, soy sauce, brown sugar, mustard, rosemary, both olive oils, and salt and pepper to taste. Whisk together until thoroughly blended. Reserve half the sauce for later use. Add the steak to the remaining sauce in the bowl and marinate for at least 1 hour or overnight.
Heat the grill as hot as it will go and cook the steak for about 2 minutes on each side for medium rare (4 minutes per side for medium). Remove the steak from the grill and place on a cutting board. Allow the steak to rest for 2 minutes and then slice into half-inch slices. Serve at once with the reserved sauce.
Photo: Ron Manville