Get Prepped: Place the 1 onion, 4 carrots, 4 celery stalks, and 2 sprigs fresh sage in the bottom of your slow cooker. Place a 6-8 pound bone in turkey breast on top of the vegetables. Blot the turkey dry with paper towels (this helps the butter and seasoning stick)
Make the Seasoning Butter: Melt 4 tablespoons butter, then whisk in 1 teaspoon poultry seasoning, 1 teaspoon garlic powder, 1 teaspoon smoked paprika, 2 teaspoons salt, and ½ teaspoon black pepper. Pour the mixture over the turkey breast.
Cook: Cover and cook on low for 6 to 7 hours, or until a digital thermometer inserted in the thickest part reads 165°F. Remove the turkey from the slow cooker and let rest for 10 minutes before slicing.
Serve: Slice and serve.
Notes
Go by temperature, not the clock. Every slow cooker runs a little different, so start checking at the 6 hour mark with a digital thermometer. Pull it around 160 to 162°F, it'll climb the rest of the way to 165°F as it rests.
You'll need a large oval slow cooker, at least 7 quarts. A round one won't fit a bone-in breast this size.
Don't toss those drippings! Strain them and use them to make gravy while the turkey rests.