Preheat the oven to 425 degrees F. Prick the sweet potatoes all over with a fork. Bake directly on the rack until tender, 50-60 minutes. Allow to cool.At the same time, cook the chicken. Place the chicken, and chickpeas - if using, in a baking dish. Add the olive oil, garlic, chipotle chiles, chili powder, onion powder, oregano, cumin, lime zest, and a pinch each of salt and pepper, gently toss to coat. Transfer to the oven and bake 20-25 minutes, until the chicken is cooked through. During the last 5 minutes of cooking, scatter the spinach over the chicken and cook 5 minutes, until wilted.Shred the chicken using 2 forks, then toss with the chickpeas, spinach and any oil left in the baking dish. Scrape the sweet potato flesh out of the peel, leaving 1/2 inch thick layer of sweet potatoes. Brush the skins with a little olive oil and bake for 5 minutes, until crisping. Remove skins from the oven and stuff with the chicken. Top with shredded cheese. Return to the oven and bake for 10 minutes, until the cheese is melted. Serve with fresh cilantro and greek yogurt, if desired.