In a mixing bowl, whisk together the fresh lime juice, lime zest, soy sauce, chopped green onions, minced ginger, and red pepper flakes.
Place the flank steak in a resealable plastic bag or shallow dish. Pour the marinade over the steak, ensuring it is well-coated. Seal the bag or cover the dish and refrigerate for at least 2 hours, or ideally overnight.
Preheat your grill or grill pan to medium-high heat.
Remove the steak from the marinade and let it sit at room temperature for about 30 minutes. Discard the marinade. Season the steak generously with kosher salt and freshly ground black pepper on both sides.
Place the flank steak on the grill. Cook for about 5-7 minutes on each side for medium-rare, or until it reaches an internal temperature of 130°F (54°C).
Once cooked, remove the steak from the grill and let it rest for 10 minutes. After resting, slice the steak against the grain into thin strips.
Serve your Lime Marinated Flank Steak warm, garnished with additional chopped green onions if desired.