Combine the garlic, rosemary, basil, and olive oil in a shallow bowl. Add the lamb, massaging the marinade into the meat. Refrigerate for at least 4 hours, or overnight.

To make the rosemary skewers, remove most of the leaves from each sprig, leaving 1 to 2 inches of leaves at the very top. Cut off the very bottom of each sprig on an angle to create a sharp point. Thread 4 to 5 cubes of lamb onto each skewer, and set aside on a platter to come to room temperature.

To prepare the couscous, combine the water and butter in a small saucepan and bring to a boil. Meanwhile, toast the couscous in a large dry skillet over medium heat until nutty brown, about 3 to 4 minutes. Transfer to a bowl.

Pour the boiling water over the couscous, cover the bowl tightly, and let sit for 15 minutes. Fluff the couscous with a fork to separate the grains. Season with salt and pepper and set aside.

Prepare a fire in an outdoor grill.

Meanwhile, heat 1 tablespoon of the olive oil in a large sauté pan over medium heat. Add the red and yellow bell peppers, onion, garlic, rosemary and basil sprigs, and cinnamon and sauté until the vegetables are soft and slightly caramelized, about 10 minutes. Transfer to the bowl with the couscous and add the remaining 2 tablespoons olive oil, the chickpeas, cayenne, and torn basil. Toss well, then taste and adjust the seasonings.

Season the lamb spiedini with salt and pepper. Grill, turning occasionally, for 6 to 7 minutes total for medium-rare. Transfer to a rack to rest.

Meanwhile, to prepare the yogurt sauce, combine the yogurt, mint, lemon juice, salt to taste, and white pepper or Tabasco in a small bowl. Transfer to a serving bowl, and sprinkle with the scallions or garlic, if desired.

Spoon the couscous onto one side of a large platter. Place the skewered lamb on the other side, flanked by the bowl of yogurt sauce.