Remove the meat from the chicken and discard the skin. Chop the meat into large chunks. Cut the chicken carcass into 2 or 3 pieces and put in a large pot with the chicken broth, leek greens, chunked celery, carrot peels, garlic, cilantro stems and 1/2 teaspoon salt. Add 6 cups water to cover, bring to a simmer over medium heat and cook until the carcass is falling apart and the broth is flavorful, 20 to 25 minutes. Strain the broth, discarding any solids, and reserve.
Wipe the pot clean and heat the olive oil over medium heat. Add the chopped leeks, celery, carrots, zucchini and yellow squash. Season with 1/2 teaspoon salt and cook until the leeks begin to wilt, 3 to 4 minutes. Add the harissa, ras el hanout and tomato paste and stir to coat the vegetables, cooking until the spices darken. Add the broth and chickpeas, cover and simmer until the vegetables are very tender, about 15 minutes. Stir in the chopped chicken, cilantro leaves and the spinach. Cook until the spinach is wilted, about 2 minutes. Serve with a spoonful of Greek yogurt on top.
Ras el hanout is a spice blend from North Africa that is frequently made with cardamom, cloves, cinnamon, cumin and pepper, among other spices. You can find it in the spice aisle of the supermarket or an online retailer.
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