Lay a chicken breast on a chopping board and use a sharp knife to make a deep pocket inside the breast. The easiest way is to push the point of a knife into the fat end, keep going halfway into the fillet. Be careful not to cut all the way through or the butter will leak … Preheat the slow cooker if necessary. Put the onion and carrot in the base of the stock pot to form a protective layer to sit the chicken on, and add 100ml boiling water. Gently ease the chicken skin away from the breast. Stir some salt and pepper into the butter and push the butter under the skin.