Simmer turkey giblets, wing tips, wishbone, vegetables, and herbs in 2 liters of water for 2 hours to make stock.
Strain the stock and preheat the oven to 200°C.
Melt 85g butter and sauté chopped onions until soft, then allow to cool.
Mix cooled onions with breadcrumbs and herbs, then weigh the stuffing.
Season the turkey and loosely fill the cavities with stuffing.
Calculate the total weight of the stuffed turkey plus 175g of butter to determine cooking time.
Melt 175g butter, soak the muslin cloth in it, and wrap it over the turkey in a tray.
Roast at 200°C for 15 minutes, then reduce to 180°C.
Continue roasting for 15 minutes per 450g of total weight, basting every 30 minutes.
Remove the muslin 15 minutes before the end of cooking and baste a final time.
Ensure juices run clear when the thickest part of the meat is pierced.
Transfer the turkey to a dish, cover with parchment and a towel, and rest for at least 30 minutes.
Deglaze the roasting tray with stock, whisking in a butter and flour paste to thicken the gravy.
Strain the gravy, bring to a simmer, then serve with carved turkey and stuffing.