Prepare the marinade by mixing the orange juice, zest, soy sauce, vinegar, toasted sesame oil, ginger, garlic, and sriracha in a Ziploc bag. Add the flank steak, seal. Refrigerate for 4-24 hours.
Preheat your oiled grill pan or grill over medium-high heat.
Remove the flank steak from the marinade and pat it dry.
Add the flank steak and grill for 5-6 minutes, then flip and grill for 3-4 minutes, or until the desired internal temperature, using a meat thermometer.
Let the meat rest for 5 minutes before slicing into thin slices AGAINST the grain.
Boil the remaining marinade in a small saucepan over high heat for 5-7 minutes until thickened, if desired.
Drizzle sauce over the sliced meat, if desired. Serve and enjoy.