Prepare your slow cooker by rubbing the sides and bottom of the stoneware with 1 tablespoon (15 ml / 0.5 fl oz) of the olive oil.
In a large skillet, heat the remaining 2 tablespoons (30 ml / 1 fl oz) of olive oil over medium-high heat. Carefully place the turkey breast in the hot skillet and brown it well on all sides, about 8 to 12 minutes. This step is key for developing a deep, roasted flavor.
Create a flavorful base by scattering the onion wedges and celery pieces in the bottom of the prepared slow cooker.
Place the browned turkey breast on top of the vegetables.
In a small bowl, whisk together the dried minced garlic, thyme, sage, salt, and pepper. Evenly sprinkle this aromatic herb mixture over the entire surface of the turkey.
Pour the water into the slow cooker around the base of the turkey, being careful not to wash off the seasonings.
Drizzle the melted butter directly over the herb-crusted top. Secure the lid and cook on LOW for 7 to 9 hours, or until the turkey is tender and cooked through.
Let the turkey breast rest for at least 10-15 minutes after cooking and before slicing. This allows the juices to redistribute, ensuring every slice is moist and tender.
Don't discard the liquid in the slow cooker!
Strain the drippings into a saucepan, bring to a simmer, and thicken with a cornstarch slurry to create a delicious, savory gravy.
