1. Combine first 5 ingredients in a small bowl.
2. Remove and discard giblets and neck from chicken. Trim excess fat. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, gently pushing between skin and meat. Rub butter mixture under loosened skin and rub over breast and drumsticks. Lift wing tips up and over back; tuck under chicken. Place lemon halves inside cavity.
3. Place chicken, breast side up, on a small rack. Place rack inside slow cooker. Cover and cook on HIGH for 3 hours or until a thermometer inserted into thigh registers 165° to 170°.
Flavor Tip: Rosemary is one of the most aromatic and pungent of all herbs. Its needlelike leaves have a pronounced lemon-pine flavor.