Grate the onion and combine it with lemon juice, olive oil, parsley, gingerroot, soy sauce, cumin, chili powder, minced garlic, dry sherry (optional), turmeric, dried oregano, and black pepper in a shallow baking dish.

Place the flank steak into the marinade, ensuring both sides are coated.

Cover the dish and refrigerate overnight, turning the meat occasionally for even marination.

Remove the steak from the marinade, setting the marinade aside in a separate container.

Grill the flank steak to the desired doneness, approximately 5 minutes per side for medium.

Transfer the grilled steak to a platter and let it rest for about 5 minutes to allow the juices to stabilize.

Strain the reserved marinade into a small saucepan, discarding any solid particles.

Bring the strained marinade to a boil over high heat and cook for 3-5 minutes until slightly reduced.

Cut the flank steak on the diagonal into thin slices.

Pour the reduced marinade over the sliced steak.

Garnish with fresh cilantro sprigs before serving.