In a medium mixing bowl, whisk together lime juice, lemon juice, grapefruit juice, rice vinegar, olive oil, 1 tablespoon soy sauce, chili garlic sauce,and honey in a bowl until the honey has dissolved.

Add ginger, salt, garlic powder, pepper and cinnamon to liquid mixute and whisk until blended.

Place flat iron steak in large resealable plastic bag, then pour the marinade over. Squeeze out excess air, seal and refrigerate 2-4 hours.

Prehear indoor or outdoor grill to medium-high heat. Remove flat iron steak from marinade, set marinade aside.

Cook steak on grill to desired degree of doneness (135 degrees for medium rare, 145 degrees for medium - for this cut of meat I wouldn't recommend much more doneness); 3-4 minutes per side.

Remove steak from grill and let rest for 3 to 4 minutes.

While steak rests, take marinade and pour into saucepan over medium-high heat.

Add sugar, 1 tablespoon soy sauce and cornstarch and whisk until thickened. Pour into serving dish. Slice steak across the grain and serve with sauce.