Place the lamb in a 4-quart slow cooker and add the garlic, tomatoes, cinnamon, sugar, 1 teaspoon salt, ½ teaspoon pepper, ½ cup water, and the tomato paste, if using.

Keeping the cilantro in a bunch, cut off the stems and tie them together with kitchen string; set the leaves aside. Nestle the bundle of stems into the ingredients in the slow cooker. Cook on the low setting for 5 to 6 hours, or until the lamb is very tender, then remove and discard the cilantro stems.

Meanwhile, put the butter, onions, and cumin in a medium skillet or sauté pan and cook over low heat, stirring occasionally, until the onion is very soft and deep golden brown, about 30 minutes. Season with salt and pepper to taste, then remove from the heat.

Twenty minutes before the end of the cooking time for the lamb, remove the lid and skim off as much of the clear fat from the surface of the stew as possible.

Reserving a few sprigs for garnish, roughly chop the cilantro leaves and stir them, along with the raisins, into the lamb; continue to cook, uncovered on the high setting, for 20 minutes.

Taste and season with salt and pepper as necessary. Spoon the couscous onto a serving platter or individual plates and top with the lamb and the caramelized onion and reserved cilantro sprigs.