Combine the Herb Butter ingredients in a bowl and mix well. Reserve half of the Herb Butter in the refrigerator for later.
Arrange the garlic cut-side down in the bottom of the slow cooker bowl. Add the sprigs of thyme and rosemary, and slice of lemon.
Using paper towels, pat the turkey dry as much as you can. Rub all over with the remaining herb butter, season generously with salt and pepper all over, then place the turkey on top of the garlic and herbs in the slow cooker bowl.
Cover and cook on HIGH for 4 hours, or LOW for 8 hours.
(About half an hour before the end of cooking time, arrange oven shelf to the lower half of your oven. Preheat your oven broiler/or grill in Australia to crisp the skin.)
Carefully transfer turkey to a roasting tray by inserting tongs on either side of the turkey into the carcass; lifting and placing straight into the roasting tray (have your tray sitting right next to the slow cooker for an easy transition). Remove 2 cups of the liquid from the slow cooker bowl, strain and reserve for your gravy. Pour any remaining juices over your turkey.
Crisp the Skin
Using the reserved herb butter, use a spoon to spread it over the top of your turkey for extra flavour (the turkey will be hot and the butter will melt nicely over the skin). Broil for about 10 minutes, OR until the skin is crispy and golden browned all over. Leave it longer if needed, but if you see it start to burn, turn the broiler off and turn the oven on to 420°F | 220°C, and continue roasting until the turkey is golden browned (about 15 minutes).
Allow it to rest for 5 minutes before carving and serving.
Melt the butter in a small pot over low-medium heat. Whisk in the flour until combined. Allow to cook for about a minute, while whisking occasionally. Pour in 1/2 cup of the Turkey liquid and whisk until it forms a paste. Add remaining liquid in 1/2 cup increments, whisking in between, until the gravy is smooth. Allow to simmer over medium heat until thickened. Take off heat and mix in the Worcestershire sauce. The gravy will continue to thicken as it cools.