Set a box grater in a 9-by-13-inch baking dish. Grate the onion quarters on the large holes, allowing the pulp and juice to fall into the baking dish. To the grated onion, stir in the pomegranate molasses, Aleppo pepper, 1 teaspoon of rosemary, 1 teaspoon salt and ½ teaspoon black pepper. Add the steaks and turn to coat. Cover and refrigerate for at least 30 minutes or up to 24 hours; flip the steaks once or twice during marination. If refrigerated for longer than 1 hour, remove the steaks from the refrigerator about 30 minutes before grilling.

Brush a 12-inch cast-iron grill pan with 2 teaspoons oil and heat over medium-high until barely smoking. Meanwhile, scrape the marinade off the steaks and pat dry with paper towels. Add the steaks to the pan and cook without disturbing until well browned on the bottom, 5 to 7 minutes. Flip the steaks and cook until the second sides are well browned and the centers reach 120°F (for medium-rare), 6 to 8 minutes. Transfer to a serving platter sprinkle with the remaining rosemary and top each with 2 pieces of butter. Tent with foil and let rest for about 10 minutes.

Transfer the steaks to a cutting board and cut into thin slices on the diagonal. Return to the platter and pour over the juices from the cutting board. Sprinkle with salt and black pepper and, if desired, drizzle with additional pomegranate molasses.

To Cook on a Charcoal or Gas Grill: Follow the recipe to marinate the steaks; if refrigerated for longer than 1 hour, remove the steaks from the refrigerator about 30 minutes before grilling. Prepare a charcoal or gas grill. For a charcoal grill, ignite three-fourths a large chimney of coals, let burn until lightly ashed over, then distribute the coals evenly over one side of the grill bed; open the bottom grill vents. Heat the grill, covered, for 5 minutes, then clean and oil the grill grate. For a gas grill, turn all burners to high and heat the grill, covered, for 10 to 15 minutes, then clean and oil the cooking grate. Scrape the marinade off the steaks and pat dry with paper towels. Brush one side of the steaks with oil, then place oiled-side down on the grill (on the hot side if using charcoal). Cover and cook until nicely charred on the bottom, about 4 minutes. Brush the side facing up with oil, then flip the steaks. Cover and cook until the second sides are nicely charred and the centers reach 120°F (for medium-rare), another 3 to 4 minutes. Transfer the steaks to a serving platter, sprinkle with the remaining rosemary and top each with 2 pieces of butter. Tent with foil and let rest for about 10 minutes. Continue with the recipe to finish and slice.