Preheat the oven to 180°C.
Using sharp kitchen scissors, cut the chicken along the backbone. (You can remove the backbone, if preferred). Flatten the chicken by placing it skin-side up on a cutting board and pushing down the breastbone. Make small incisions in the legs.
Grate the lemon zest then cut the lemon in half and juice it. Then cut the lemon into rings.
Mix the garlic with the butter or margarine and the leaves of 4 sprigs of thyme. Add the lemon zest, half of the juice, then salt and pepper.
Rub the chicken all over with this mixture, then under the skin and push some into the cuts too.
Put the lemon rings on top of a rack, add the remaining thyme sprigs. Place the chicken on top.
Put the rack in a roasting tray and add some white wine or chicken stock. Add the onion halves.
Roast in the middle of the oven for 40 minutes, basting it regularly. Pour over the remaining lemon juice and roast for another 5-10 minutes or until the skin is golden and the chicken has cooked through (the juices should run clear when pierced with a knife).
Rest the chicken while you make gravy from the jus.
Carve the chicken and serve with the gravy and side dishes of your choice.