In a medium bowl, whisk chili sauce, fish sauce, sesame oil, ginger, and garlic. Set aside a few tablespoons of the mixture for brushing the steaks during grilling.
Score flank steak and place in a shallow dish. Pour remaining marinade over the steak, and turn to coat. Cover, and marinate in the refrigerator at least 3 hours.
Preheat an outdoor grill for high heat.
Lightly brush the grilling surface with oil. Grill steak 5 minutes per side, or to desired doneness, brushing frequently with the reserved marinade mixture.