Preheat the bottom part of the couscoussier on medium heat for a couple of minutes. Put in olive oil, chopped onion, garlic, and seasonings. Stir and sauté for 5 min, or until the onions soften. Add chicken, brown for 5 min, flip, and brown for a few more minutes. Add 1 cup water, cover, and cook for 15 min. Add cabbage and 1/2 cup of water, and cover. Meanwhile, grease the top part of the couscoussier: the steamer.
In a large bowl, combine dry couscous grains with 1 tsp salt and 2 Tbsp olive oil, and mix with your hands until it's all combined. Add 1 cup of room temperature water over the couscous mixture, and let it absorb water for 10-15 minutes without mixing it. Meanwhile, add cabbage, fava beans (if using), and 1 cup of water to the bottom pot and cover.
Fluff the couscous with your hands and place it on the lightly greased steamer; be careful not to press the couscous grains down it. Allow the couscous to steam for about 15 minutes, uncovered. Pour the couscous back into the large bowl. Add 1 cup of water and 1 Tbsp of olive oil, and mix with a wooden spoon. Allow the couscous to cool for 5-10 minutes before mixing it with the hands to separate the grains.
Put the couscous back into the steamer a second time, and let it steam, uncovered. Add the sweet potatoes, potatoes, and carrots to the bottom pot and cook for 10 minutes. Add more water, but just enough for the veggies to cook if needed. Add zucchini and chickpeas, and cook for five more minutes. (for this step, the total cooking time should be 15 minutes for both couscous and veggies)
Pour the couscous back into the large bowl, add unsalted butter and salt to taste, and fluff with your hands. If couscous is too hot, you may use a wooden spoon to fluff it. Your couscous should be tender and fluffy by now!