Photograph: Tara Fisher
Preheat the oven to 200°C, fan 180°C, gas 6. Put the onion slices in the centre of a roasting tin and add some of the herbs. Pop the remaining herbs in the chicken cavity, rub the skin with olive oil, season and sit the chicken on the onion. Roast for 1 hour 20 minutes - 1 hour 30 minutes, or until cooked through.
Tip any juices from the chicken cavity into the roasting tin, then put the chicken on a carving board, cover with foil and leave to rest. Remove any burnt bits of onion from the roasting tin, skim off the fat and place over a low heat. Stir in the flour with a wooden spoon, scraping the tin, then stir in the wine, bring to the boil and simmer for 2 minutes. Stir in the stock and simmer for 5 minutes. Strain 750ml into a measuring jug to cool before freezing. Serve the remaining gravy with the chicken to eat now.
To freeze, once the reserved gravy is cold, pour into a freezerproof container and cover.
To reheat, defrost in the fridge for 24 hours. Bring to the boil in a pan, adding juices from your roast turkey; simmer for 5 minutes. Check the seasoning before serving.
Get ahead: make the gravy up to 3 months ahead and freeze.