Pat the turkey breast dry with paper towels.
Lay the breast skin side down on a cutting board. Season the meat side with 2 tablespoons of the turkey rub
In a small bowl, mix the remaining turkey rub with the butter and sage until well combined.
Loosen the skin on the turkey breast without detaching it. Stuff half of the seasoned butter between the skin and meat, spreading as evenly as possible.
Spread the remaining seasoned butter on top of the skin.
Pour turkey or chicken stock or broth in the slow cooker.
Transfer the turkey breast to the slow cooker, skin side up. Sprinkle more seasoning on top if desired.
Cover and cook on low for 3-4 hours. Use the meat thermometer ensure a temperature of 160°F – 162°F. It will raise to the recommended 165℉ as it rests. Add additional cook time if needed.
Once cooked, transfer the turkey breast from the slow cooker to a cutting board and rest 10-20 minutes (while you make the gravy).
Skim the remaining liquids in the crockpot of any solids and discard. Separate and discard the fat if desired.
In a small mixing bowl, whisk together cornstarch and 2 tablespoons water or broth. Add the mixture to a medium skillet or saucepan along with the strained juices from the crockpot on medium-high heat. Bring the mixture to a boil, then lower to simmer until thickened to your desired texture. Taste and adjust the seasonings as desired.
Slice the turkey breast in 1/2-inch slices, then place on a serving plate. Garnish the plate as desired and serve the gravy on the side.