Pat the chicken dry inside and out.
Mix the salt, black pepper, lemon zest, rosemary, garlic and olive oil and rub it over the entire bird.
Spread the onion slices in a 6-quart slow cooker, pour in the broth and set the chicken breast-side up over them.
Cover and cook on low until the breast reaches 165°F and the thigh reaches at least 175°F, 5 to 5 1/4 hours.
Move the chicken to a board and rest it for 15 minutes before carving.
Skim the onion juices, stir in the fresh lemon juice and spoon them over the carved chicken.