Place the onion, carrots and celery in the bottom of a 6 quart crock pot. Pour the chicken broth on top.
In a small bowl, combine the ingredients for the dry rub.
Remove the skin from the turkey breast as it will not get crispy in the crock pot.
Rub the dry rub mixture all over the turkey breast and place breast side down in the crock pot, making sure that it sits up on the vegetables. Add the cubes of butter on top of the turkey breast.
Cook on high for 1 hour then turn to low and cook 4-5 hours more or until a thermometer inserted in the center of the breast reaches a temperature of 165 degrees.
Remove the turkey breast from the oven and allow to rest on a cutting board for a few minutes.
Strain 2 cups of the cooking liquid from the crock pot and discard any solids.
Melt the butter in a medium skillet over medium heat. Whisk in the flour and then slowly stream in the cooking liquid, whisking constantly, until a gravy of your desired consistency is reached. Season to taste with salt and pepper.
Cut the turkey breast into slices, plate and drizzle the gravy on top.