Spray the inside of a 6 quart slow cooker with cooking spray.
Rinse and pat dry the turkey breast. Rub with 3 Tbsp of softened butter all over and under the skin. Pull back the skin and sprinkle the breast meat liberally with seasoning. Pull the skin back over the breast and season the exterior of the bird inside and out. Sprinkle with salt and pepper.
Arrange the chopped celery, carrot, onion and bay leaves in the bottom of the slow cooker. Drizzle with chicken broth.
Center the turkey breast in the slow cooker and dot the top with the remaining 2 Tbsp of softened butter.
Cook on high for 30 minutes, then on low for 6-7 hours, depending on your slow cooker. Check for doneness at 6 hours. Poultry should be cooked to an internal temperature of 165°F.
Remove to a platter and tent loosely with aluminum foil. Rest for at least 15 minutes before carving.
To make gravy: Strain the broth pour into a stove top pan. To thicken whisk in a flour or cornstarch slurry. Adjust the seasoning, to your taste.
Keep warm until serving.