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.