Break the celery into half or less so it fits in the pan. Use the flat side of a knife and squash the garlic, you don't have to be precise, just break the outer skin. Squeeze the lemon juice into the pan and add the rest of the lemon outer.
Put all the ingredients into the pan .
Add boiling water to the pan so it comes up 1 inch above the chicken breasts
Place the pan on a low simmer, so you do not see bubbles. IT MUST NOT BOIL! Keep on a gentle heat for 20 - 25 minutes, then switch off the heat and leave to cool to touch.
Once the chicken is cool enough to handle, remove from the pan and either store in the fridge if using later (place in the fridge once the chicken has completely cooled), or slice and add to a salad, use in sandwiches, pizza toppings etc.