In a large pot over medium-high heat, cook oil and garlic until fragrant, 1 minute. Add ras el hanout and chiles and cook, stirring, 1 minute. Add 1/2 tsp. salt, 3 cups broth, 2 cups water, carrot, parsnip, potato, turnip, sweet potato, and zucchini. Bring to a boil. Add chickpeas, lower heat, and simmer vegetables until just tender, 15 minutes. Add tomatoes, if using, and parsley. Cook vegetables until tender, 3 to 5 minutes. Add salt to taste.
Bring remaining cup broth and 1 1/4 cups water to a boil in a medium pot. Add couscous, cover, and turn off heat. Let sit 5 minutes, then fluff with a fork.
Remove chiles from vegetables and discard. Serve vegetables over couscous, garnished with preserved lemon if you like.
Ras el hanout: Mix together 1/2 tsp. each ground ginger, turmeric, and coriander; 1/4 tsp. each freshly ground black pepper, cinnamon, nutmeg, and saffron (optional); 1/8 tsp. each ground allspice, cloves, and mace; and crushed seeds from 6 cardamom pods (optional).
Note: Nutritional analysis is per serving.