If you have time, the day before you roast your chicken, take it out of any packaging, put it on a plate and sprinkle it inside and out with salt. Leave it in the fridge overnight, loosely wrapped in kitchen paper. An hour before you want to start roasting the chicken, remove it from the fridge so it can come up to room temperature.
To make the sage and onion stuffing, melt the butter in a frying pan, add the onion and cook it over a low heat until it’s very soft and translucent. Tip the onion into a bowl and let it cool, then add the lemon zest, sage and breadcrumbs. Season well with salt and pepper, then add the egg and mix thoroughly.
Preheat the oven to 220°C/Fan 200°C/Gas 7. Weigh the stuffing and add it to the weight of the chicken so you can work out the cooking time. The chicken will need 20 minutes for every 500g, plus 15 minutes at the high temperature to begin with. Fill the cavity of chicken with your chosen stuffing, making sure you leave a little space between the stuffing and the top of the bird, just to let the air circulate. If you have any stuffing left over, put it in the neck end.
Put the chicken in a roasting tin, rub the butter all over it, season, then squeeze the juice from the lemon half over the top. Add the white wine or vermouth with 100ml of water, or simply use 200ml of water.
Put the chicken in the oven and roast it for 15 minutes. Turn the heat down to 180°C/Fan 160°C/Gas 4 and continue to roast for the time you’ve worked out. You can test for doneness in a variety of ways. If you have a probe or meat thermometer, the stuffing and the thickest part of the chicken should be 75°C. Or if you pierce the thickest part of the thigh with a skewer, the juices should run clear. The legs should also feel loose when you wiggle them.
Once the bird is cooked, remove it from the tin and cover it with foil to keep it warm while it rests. Pour the juices from the tin into a jug and strain off any fat, then set the juices aside. To make the gravy, sprinkle the flour into the roasting tin and place the tin over a low heat. Stir the flour into the scrapings at the bottom of the tin to make a roux, then pour in the white wine. Let this bubble up, then gradually add in the defatted pan juices and the chicken stock. At this point you can transfer the gravy to a saucepan if you like – the roasting tin should look clean. Simmer until you have a thin, tasty gravy to serve with the chicken and all the trimmings.