Place the butter, garlic, lemon juice and zest, rosemary, sage, ground black pepper and soy sauce in a small bowl. With a fork, as in mashing mash potatoes, combine all the ingredients. The lemon juice and soy sauce will not be completely incorporated (see photo above).
Place some of the herb butter under the skin, rub the remaining herb butter all over the breast. Season lightly with salt and ground black pepper. Reserve the remaining liquid from the herb butter mixture (it won't be much but we will use it)
Place the onion, celery, carrots, garlic clove and thyme at the bottom of the slow cooker. Place the turkey breast skin side down on top of the vegetables. Pour the remaining liquid from the herb butter in the turkey cavity.
Cover the slow cooker and cook on high for 1 hour. After that on low for 5 to 6 hours or until the internal temperature reaches 165 degrees Fahrenheit.
Remove the turkey from the slow cooker and place it on an aluminum foil lined sheet pan. Let it rest for 10 minutes.
Crisp the Skin for a Picture Ready Bird
Preheat the broiler to high and place the oven rack about 12 inches from the heat source.
Place the turkey breast under the broiler and broil for 4 to 5 minutes. Keep an eye on it! It gets golden very fast! Remove from the oven, carve and serve with the gravy.
Make the Gravy
Strain the liquid from the slow cooker into a measuring cup. You should have about 2 cups of liquid. If not, add chicken/turkey broth or water to complete the two cups.
In a small saucepan over medium heat, melt the butter. Whisk in the flour and mix well to combine. Cook for about 2 minutes. Stir in about half a cup of the cooking liquid into the flour mixture, mixing well. It will look like a paste. Slowly add the remaining liquid whisking constantly.
Simmer on low heat for about 5 minutes. Remove from the heat and season to taste. Serve immediately or keep warm in a thermos.
Notes
Thaw out the turkey completely before cooking it. Frozen turkey can take up to 48 hours to thaw out in the refrigerator.
The turkey is ready when a meat thermometer inserted in the thickest part of the breast registers 165 degrees Fahrenheit. (as per USDA safety guidelines). You can remove the turkey from the slow cooker when the temperature reaches 160 degrees Fahrenheit. The temperature will continue to rise while resting.
Always allow the turkey to rest, tented loosely with aluminum foil, before carving it.
Don't cook the turkey on high for any longer than 1 hour. Cook the turkey on low until the internal temperature reaches 165 degrees Fahrenheit.