Place the carrots, celery, onions and garlic in the slow cooker. Pour in the wine and chicken broth.
In a small bowl combine 4 tablespoons of the butter, sage, celery seed, salt and pepper. Spread the mixture over the turkey breast and place the turkey in the slow cooker on top of the vegetables.
Cover and cook on low for 6 hours, or until the turkey is fully cooked with an internal temperature of 165°F.
Optional Broiling step: The turkey is ready to serve at this point, though the skin will not be crispy. If you wish to crisp up the skin, just before the turkey is cooked, preheat the broiler. Transfer the turkey to a roasting or broiling pan and broil the turkey until the skin gets browned; you may need to turn the turkey breast on one side and then the other to get the most even browning. Watch it carefully so that it doesn’t burn.
Meanwhile, make the gravy. Strain the juices from the slow cooker with a strainer, discarding the solids.
Melt the remaining 2 tablespoons butter in a small saucepan over medium heat. Whisk in the cornstarch or flour and continue whisking until it turns golden brown, 1 to 2 minutes. Gradually whisk in the strained pan drippings. Bring the gravy to a boil, whisking frequently until thickened 6 to 8 minutes. Transfer to a pitcher or gravy boat.
Let the turkey breast sit for 15 minutes before slicing and serving with the warm gravy.