Put the flank steak in a large self-sealing plastic bag. Whisk the sherry, soy sauce, brown sugar, Essence, garlic, tomato paste, and pepper in a medium bowl. Pour into the bag and seal. Refrigerate for at least 4 hours, and up to 24 hours.

Preheat a gas or charcoal grill.

Remove the steak from the marinade and pat it dry. Grill the steak for about 6 minutes on each side for rare. While it cooks, pour the marinade into a small heavy saucepan and bring to a boil over high heat. Lower the heat to medium-low and simmer until reduced and thickened, about 10 minutes.

Transfer the steak to a cutting board and let stand for 5 minutes before slicing.

Cut the steak across the grain into thin diagonal slices, and serve with the sauce on the side.