Mix the herb butter ingredients together.
Pop the onion chunks into the bottom of the slow cooker and set the chicken on top. (I like to cut off and remove and string holding the bird. I think this makes it cook more evenly.)
Spread the herb butter over the top and sides of the chicken.
Slice one of the lemons in half and push in around the edges of the bird. Add 100ml/0.5cup of boiling water. Pop the lid on and cook for about 5 hours on LOW.
The chicken is cooked when if you wiggle a leg bone, it feels very loose. You can now serve (I tend to remove the skin and portion up if serving like this as the skin doesn't look/taste very good.) OR if you'd prefer to serve as a regular whole bird roast:
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