Combine chopped onions, ginger, garlic, olive oil, lemon juice, soy sauce, sherry, cilantro, chili powder, cumin, harissa, salt, pepper, turmeric, marjoram, and saffron in a bowl and mix well to make the marinade.

Pierce flank steaks all over with a sharp fork on both sides.

Place the steaks in a large zip-top plastic bag.

Pour the marinade over the steaks, seal the bag, and squish everything to coat the meat evenly.

Refrigerate and marinate overnight, turning the bag occasionally.

Remove the marinated steak from the refrigerator and allow it to come to room temperature.

Light a charcoal fire or preheat a gas grill to high heat.

Oil the grill's cooking surface.

Let the coals burn down to medium-hot or adjust the gas grill burners to medium-high.

Pat the steaks dry with paper towels.

Place the steaks on the hot grill and cook for 5-6 minutes per side for medium-rare, or longer for desired doneness.

Remove the grilled steaks to a platter, cover loosely with aluminum foil, and let rest for 5 minutes.

Slice the rested steak across the grain into 1/4-inch-thick slices.

Serve immediately.