In a large bowl, thoroughly mix all the ingredients except for the flank steak and the garnish. Pour half the marinade into a resealable 1-gallon plastic bag and add the flank steak. Marinate in the refrigerator for at least 8 and up to 24 hours.
Pour the remaining marinade into a saucepan over medium-high and simmer until it is reduced by half, about 10 to 15 minutes. Season to taste.
Preheat a grill to high or heat a skillet over high heat. Remove the flank steak from the marinade and wipe off the excess liquid with paper towels. (Discard the meats marinade.) Grill or pan-sear both sides, then lower the heat to medium and cook to medium rare (135F), 8 to 10 minutes.
Let the flanks steak rest, covered with foil, for 5 to 10 minutes. Cut the steak on the bias across the grain and serve with the reduced marinade. If desired, garnish with celery and tomatoes.