Cut the chicken in half (or ask your butcher to do this for you), rinse under cold water and remove the fat. Combine the chicken, onions, celery, carrots, bay leaves, peppercorns and 3 litres of water in a large saucepan. Bring to the boil; reduce the heat and simmer, uncovered, for 45 minutes. Remove the chicken from the pan.
Strain stock into a medium saucepan (you will need 1.5 litres of stock). Keep stock hot over a low heat while preparing the risotto.
When cool enough to handle, remove the chicken flesh from the bones; discard bones. Cover the chicken with foil to keep warm.
Heat the butter in the same large saucepan, cook the extra onion over a medium heat until soft. Add the garlic; cook, stirring for 1 minute. Add rice and stir to coat in the onion mixture.
Add 1 cup of simmering stock; cook, stirring, over medium heat, until stock is absorbed. Continue adding stock mixture in 1 cup batches, stirring until absorbed between additions. The total cooking time should be about 25 minutes or until the rice is almost tender.
Stir through the parmesan and the shredded chicken. Cover, stand, for 5 minutes. Serve risotto sprinkled with green onions, grated lemon and extra parmesan cheese, if desired.