Coat an unheated large nonstick skillet with nonstick cooking spray. Preheat over medium heat. Brown chicken, half at a time, in hot skillet. In a 3-1/2- or 4-quart slow cooker, combine frozen artichoke hearts, garlic, and onion. In a small bowl, combine broth, rosemary, lemon zest, and pepper. Pour over vegetables in slow cooker. Add browned chicken; spoon some of the garlic mixture over chicken.