How to Make It
In a small bowl, mix 1/2 cup of the olive oil with the garlic, lemon juice, cumin, cardamom, Aleppo pepper, 1 teaspoon of black pepper and 1 1/2 tablespoons of salt. Set the lamb on a rimmed baking sheet and coat it completely with the marinade. Cover and refrigerate overnight or, preferably, for 2 or 3 days.
Light a grill. In a large skillet, heat 1 tablespoon of the olive oil. Add the bell pepper strips, cover and cook over moderately low heat, stirring occasionally, until tender, about 10 minutes. Season the peppers with salt and pepper and transfer them to a serving bowl. Heat the remaining 1 tablespoon of olive oil in the skillet and add the onions. Cook over moderate heat, stirring occasionally, until lightly browned, about 10 minutes. Season the onions with salt and pepper and transfer them to another serving bowl.
Grill the butterflied lamb over moderately high heat, turning and shifting often, until an instant-read thermometer inserted into the thickest part of the meat registers 130° for medium-rare, about 25 minutes. Transfer the lamb to a carving board and let rest for 10 minutes. Thinly slice the lamb across the grain and serve with the peppers, onions, Yogurt-Tahini Sauce and Syrian Pita Bread.