How to Make It
In a large enameled cast-iron casserole, melt the butter. Add the tomatoes, lamb, onion, cumin and 1 teaspoon each of salt and pepper and cook over moderate heat, stirring occasionally, until the onion is softened, about 10 minutes. Stir in the 1 tablespoon of harissa and 4 cups of water and bring to a simmer. Cover and gently simmer over low heat until the lamb is almost tender, about 45 minutes.
Meanwhile, in a large bowl, toss the couscous with the vegetable oil. Stir in 6 cups of the water. Drain the couscous in a fine sieve and return it to the bowl, fluffing it with your hands to remove any lumps. Cover the couscous with plastic wrap.
Add the carrots, turnips and cabbage to the casserole. Set a large bamboo steamer, lined with cheesecloth, over the casserole and bring the stew to a brisk simmer over moderate heat. Spread 1/2 cup of the couscous in the steamer and steam for 5 minutes. Gradually sprinkle the remaining couscous on top, mounding it slightly, and steam it over the stew for 20 minutes.
Scrape the couscous into a large, shallow bowl and sprinkle on the remaining 3/4 cup of water and 1 teaspoon of salt. Fluff the couscous with a wooden spoon and let cool for 10 minutes, stirring occasionally with your hands to remove any lumps.
Add the eggplant, butternut squash and zucchini to the casserole and bring to a simmer over moderate heat. Set the steamer over the stew once again and gradually sprinkle the couscous into it, mounding it slightly. Let the couscous steam over the stew for 15 minutes longer.
Transfer the couscous to a large serving bowl; form a well in the center. Season the lamb with salt and pepper, spoon it into the well and arrange the vegetables around it. Pour the sauce around the couscous and serve with the harissa.
Harissa, a North African chile paste, is available at Middle Eastern markets and specialty food stores.