Optional: If you like you can mash or process the vegetables and add to the gravy for extra flavour.
- Mix all ingredients of Seasoning Rub together in a small bowl.
- Spray inside of slow cooker with cooking spray. Place onion, carrot and celery in bottom of cooker.
- Pat turkey breast dry with paper towels. Rub liberally with Seasoning Rub. Place on top of vegetables in slow cooker.
- Cook on low for 5-7 hours or until an instant-read thermometer inserted into thickest part of breast reads 165ºF (74ºC). Remove and allow to rest for 15 minutes.
- To make Gravy: Pour the liquid and vegetables remaining in the slow cooker through a sieve into a saucepan. If there is not 2 cups (500 mL) of liquid, use poultry broth to make up the difference. Whisk in flour and black pepper. Bring to boil over medium-high heat whisking constantly. Turn down heat while maintaining a simmer and cook until thickened, about 2 minutes.