In a large resealable plastic bag or shallow dish combine the orange juice, grated ginger, olive oil, ground coriander, sea salt, and several grinds of black pepper.
Add the flank steak to the bag, squeeze out excess air, and seal; turn the steak to coat evenly in the marinade.
Refrigerate the steak for at least 4 hours or overnight to marinate.
When ready to cook, preheat a grill or grill pan to high heat.
Remove the steak from the marinade and discard the marinade. Grill the flank steak 3–5 minutes per side, depending on thickness, until desired doneness.
Transfer the cooked steak to a cutting board and let rest for 10 minutes.
Slice the steak thinly across the grain and serve immediately.