How to Make It
Combine first 6 ingredients. Rub mixture on outside and under skin of chicken. Stuff cavity with lemon, garlic, and 4 onion quarters.
Place 2 inverted shallow ramekins in a 6-quart slow cooker. Place carrots, celery, and remaining onion quarters around ramekins; add water. Place chicken, breast side up, on ramekins. Cover; cook on HIGH until a thermometer inserted into the breast registers 165°F, 3 1/2 hours. Transfer chicken to a cutting board; let rest 15 minutes.
Preheat broiler to HIGH with oven rack 6 inches from heat. Remove ramekins. Pour stock mixture through a wire-mesh strainer into a bowl; discard solids. Reserve stock for later use. Cut chicken into 6 pieces. Place, skin side up, on a rimmed baking sheet. Broil until skin is golden and crispy, 4 to 6 minutes.