In a small bowl, combine 1/4 cup extra-virgin olive oil, 6 cloves minced garlic, 2 tablespoons minced fresh rosemary, 1 1/2 teaspoons kosher salt, and 1/2 teaspoon black pepper; stir to combine.
Place 12 lamb porterhouse chops in a non-metallic dish or a zip-top bag, pour the marinade over the chops, and turn to coat. Seal or cover and refrigerate to marinate for at least 30 minutes or up to 24 hours.
About 30 minutes before cooking, remove the chops from the refrigerator and allow them to come to room temperature.
Preheat a large skillet or griddle pan over medium-high heat until hot.
If the pan is crowded, cook the chops in batches. Add chops to the hot pan (no additional oil needed) and cook about 3 minutes on the first side, then flip and cook about 3 minutes on the second side.
Transfer the cooked chops to a plate, tent loosely with aluminum foil, and let rest 5–10 minutes; reserve any juices that collect on the plate.
While the chops are resting, return the skillet to medium heat, add 1 tablespoon butter, and swirl to melt, scraping up any browned bits and mixing with the pan juices.
Spoon the melted butter and pan juices over the rested lamb chops before serving.