Soaked the chickpeas overnight in a pot, then drain and rinse well. (Alternate method: Boil beans for 2 minutes, then remove from heat and leave covered for 1 hour, then drain.)
Prepare the Chicken
In the skillet, warm the olive oil over medium heat. Season the chicken with black pepper and place the pieces in the skillet skin side down and cook just until brown, about 8 to 10 minutes per side. Place the chicken pieces in the bottom of the slow cooker and put the lid on to keep the chicken warm. Drain most of the oil out of the pan, leaving just enough to cover the bottom. Add the onions and sauté until browned, about 10 minutes. Add the garlic and cook about 2 to 3 more minutes. Pour the onion and garlic mixture over the chicken in the slow cooker and add the chickpeas. Add the harissa paste, salt, and tomato paste.
Add the sweet potatoes, celery root, carrots, red potatoes, and the preserved lemon, then pour the water over it all, stirring the vegetables gently to make sure they are all covered. Cover and set the temperature to low and allow to cook for 8 hours.
Before serving, remove the lemon and use tongs to fish out the chicken pieces, then remove the bones and skin and put the meat back into the pot. You can also serve the pieces whole on the side for those who prefer it that way. Serve with couscous, quinoa, rice, or flatbread.