Season steak liberally with salt and pepper. Place steak in a large resealable bag and pour in Mojo sauce. Seal bag, removing as much air as possible, and toss to evenly coat steak in sauce. Let marinate for 1 hour at room temperature.
Light one chimney full of charcoal. When all the charcoal is lit and covered with gray ash, pour out and arrange the coals on one side of the charcoal grate. Set cooking grate in place, cover gill and allow to preheat for 5 minutes. Clean and oil the grilling grate. Remove steak from marinade and pat dry. Place steak on hot side of the grill and cook until well seared on both sides, about 3 minutes per side. Move to the cool side of the grill, cover, and continue to cook until the center of the steak registers 125°F on an instant-read thermometer for medium-rare, or 135° for medium. Transfer to a cutting board and let rest for 10 minutes.
While steak is resting, brush onion slices all over with olive oil and season with salt. Place over the hot side of the grill and cook until softened and charred, about 3 to 5 minutes per side. Remove to cutting board.
Carve steak against the grain into 1-inch slices and serve with grilled onions.