First, make the rub by mixing all the ingredients listed under “rub” together in a small bowl.
Then, detach the skin from the chicken by delicately running your fingers under it. Start at the butt end of the bird and slip your fingers between the skin and meat, starting with the breasts first, then go ahead and gently, carefully, move all the way to the front of the bird, and under, and over to the legs too, separating the skin from the meat as you go. Go as gently as you can so as not to tear the skin.
Now, using your fingers still, go ahead and rub about ¾ of the rub directly on the meat, under that now loosened skin. Try to cover as much of the meat as you possibly can. Use what’s left of the rub to rub over the skin, from the outside this time.
Stuff your chicken with the lemon wedges, garlic and fresh thyme and then tie up the legs with a piece of butcher’s twine.
Add the chopped leek, garlic and fresh thyme at bottom of your slow cooker to make a bed for the chicken, then add the lemon halves at both ends so you can place your chicken over them. Sprinkle with salt and pepper, and, if time allows, place in the refrigerator overnight for flavors to develop.
In the morning, take your chicken out of the fridge and cook on low for 8 hours or high for 5 hours.