Heat oil in a large pot over medium heat. Add lamb; cook and stir until browned on all sides, 3 to 5 minutes. Add chickpeas, tomatoes, tomato puree, onion, ras el hanout, saffron, salt, and pepper. Cover with 1 1/2 quarts water. Bring to a boil; reduce heat and simmer until flavors combine, about 30 minutes.
Pour couscous into a very large bowl. Cover with 1 1/2 cups water. Let soak for 15 minutes. Drain and transfer to a couscous steamer insert.
Stir zucchini, turnips, carrots, and celery into the pot. Set couscous steamer on top; cover and steam until tender, about 45 minutes.
Spoon couscous back into the bowl. Stir in 1/2 cup plus 2 tablespoons butter and fluff with a fork. Serve lamb and chickpea stew on top.