Preheat oven to 350 degrees F and have ready a large roasting tin.
Wash chicken and pat dry with paper towels. Place in roasting tin and set aside.
In a medium skillet set over medium to high heat, add the olive oil and onions. Cook, stirring regularly, for about 5-7 minutes or until onions are soft. Set aside.
In a food processor, add the slices of bread and pulse until you have coarse bread crumbs. This takes about 20 seconds or so. Transfer the bread crumbs to a medium bowl. Add the melted butter, onion, thyme, lemon zest, parsley, salt and pepper. Stir really well or mix with hands to combine.
Place the stuffing inside the cavity of the chicken. Tie the chicken legs together with kitchen twine. Rub the extra tablespoon of butter over the skin of the chicken.
Place in oven and roast for about 1/2 hour. Remove from oven and loosely wrap in aluminium foil. Put back in the oven for a further hour. Take chicken out of oven and baste it with the pan juices. Remove foil and place back in oven for a further 1/2 hour or so. Juices will run clear when chicken is cooked.
Remove from oven. Cut the string off the legs and with a spoon scoop out the stuffing and transfer to a serving dish. Serve after letting stand at room temperature for about 10 minutes.