1. Around 2 hrs before you want to roast your turkey, take it out of the fridge and let it stand somewhere safe so it can come to room temperature. Pluck out any very large, visible black quills. Smaller quills will melt during cooking. Take the giblets out of the turkey and rinse them.
2. While the turkey comes to room temperature, peel and finely chop 1 onion. Trim, peel and dice the carrot. Trim and dice the celery stick. If you have kitchen string, tie the bundle of bouquet garni herbs together and remove any plastic or elastic band ties.
3. Warm a medium pan over a medium heat and add 1 tbsp oil. Add the giblets. Fry for 3-4 mins, turning occasionally, till browned. Meanwhile, fill and boil your kettle.
4. Add the chopped onion, carrot and celery to the pan with the bouquet garni. Fry the veg and giblets for 10 more mins, stirring occasionally, till soft. Pour 1.2 ltrs hot water into the pan and bring to the boil, then pop on a lid and reduce to a gentle simmer for 45 mins. After 45 mins, strain the stock into a bowl through a fine mesh, heatproof sieve. Discard the giblets, veg and herbs. Set aside till you’re ready to make your gravy (you can make the giblet stock up to 3 days ahead and keep it in a sealed tub in your fridge, if you’d like to get ahead).
5. When you’re ready to roast your turkey, heat your oven to 180°C/Fan 160°C/Gas 4. Peel and finely slice the remaining onions. Spread them out in a layer in a roasting tin and season with salt and pepper. Weigh your turkey to check how much it weighs exactly, then lay it on top of the onions, breast-side down. Pour 500ml hot water into the tin around the turkey, but not over it. Put the turkey in the oven on the middle shelf. Roast for 1 hr (this is the same, whether your bird weighs nearer 4kg or 5kg).
6. After 1 hr, take the turkey out of the oven and carefully turn it over so it’s breast-side up (make sure to protect your hands from the heat and have some sturdy spatulas to help with turning it). Make sure no onions are clinging to the turkey breast but are left in the tin. Dot 60g of the butter over the turkey breast and season with salt and pepper.
7. Use a sharp knife or skewer to slice right through the skin between the leg and breast to allow the juices to drain out and the thighs to cook. Top up the liquid in the tin with 250ml hot water. Return the turkey to the oven and roast for another 1 hr. If your turkey is closer to 4kg, then you only need to roast it for 45 mins more.
8. To check the turkey is ready, insert a skewer into one of thighs. If the juices run clear, it’s ready. Transfer to a board. Wrap loosely in foil and rest for 1 hr. You can also check the temperature with the meat thermometer – see our guide to using it.
9. Pour any juices from the roasting tin through a sieve into a bowl. Keep the onions for later. Let the roasting tin juices sit for 15-30 mins so any fat rises to the surface. Skim the fat off the juices. Pour them into a large measuring jug and add the giblet stock. You want around 1.5 ltrs of stock, so if it is a little under, top up with water.
10. To make the gravy, melt 40g of butter in a medium-sized pan, then add the cornflour and cook, stirring, for 2 mins, till combined and smooth. Crumble in the vegetable stock cube and whisk that into the roux till combined. Slowly pour or ladle in the giblet stock, a little at a time, and whisk to smoothly combine it with the roux. When it has all been added, bring up to the boil and then reduce the heat and simmer for 15-20 mins till it has reduced by around a quarter. Taste the gravy and add more salt and pepper, if you think it needs it. Transfer to a gravy boat.
11. Carve the turkey and serve it with the gravy, the onions from the roasting tin and cranberry sauce.