Get a good roasting pan and fill the bottom with the chopped onion, garlic and rosemary – this will create a tasty trivet for your chicken to sit on.
Mix the butter, thyme, salt and pepper then rub underneath the skin on the chicken.
Slice the lemon and put the slices in the cavity of the chicken.
Place the chicken in the pan and cover with tin foil. Place in the pre-heated oven for 1 hour.
After an hour take off the tin foil and put back in the oven for 30 minutes or until the juices run clear (you can check this by piercing the thigh with a skewer, the juices should run out – if there’s blood in them cook for longer).
When you take it out you should have a lovely crispy skin on the chicken and some juices in the trivet that you can use to make gravy.