Place onion, celery, broth, 1/2 teaspoon salt, pepper and sage in a 5-quart slow cooker. Add chicken, cover and cook on low for 3 to 4 hours, checking from 3 hours and removing chicken as it reaches 165°F.
During the final half hour, cover potatoes with the measured water in a saucepan. Bring to a boil and simmer for 15 to 20 minutes until tender, then drain thoroughly.
Mash potatoes with the potato butter, milk and remaining salt. Cover to keep hot while finishing the gravy.
Strain the chicken cooking juices into a heatproof jug and skim off fat. Measure exactly 2 cups, adding only as much of the additional 1/2 cup broth as needed to reach that amount; reserve any excess cooking liquid for another use.
Melt gravy butter in a saucepan and whisk in flour. Cook over medium heat for 3 to 4 minutes until light brown, then gradually whisk in the measured cooking liquid and Worcestershire. Simmer for 4 to 5 minutes until smooth and thickened.
Pull chicken into large pieces and heat in the gravy for 2 to 3 minutes until it reaches 165°F. Divide potatoes among 6 plates and spoon the chicken and gravy over them.