Rub the oil all over the turkey breast and then sprinkle with the spice mixture.
Place the turkey in the bowl of the slow cooker with the water, celery stalk, and onion wedge.
Cook on low for 5-6 hours until tender. Remove meat from bone and slice.
For the gravy:
Whisk the cold water and flour together in a glass or small bowl and set aside (make sure there are no lumps. I recommend pouring it through a mesh strainer).
Discard the celery and onion and strain the turkey drippings from the slow cooker into a sauce pan and bring to a low simmer over medium heat.
Stream in the flour mixture while whisking. Continue to cook while mixing until thickened to desired thickness. If it gets too thick or tastes too salty just add a little water.