Wash the turkey and pat dry. Rub the outside with butter and season with salt and pepper. Distribute butter between the breast and under the skin.
Quarter the apples and lemon. Stuff the turkey with apples, thyme sprigs, and lemon pieces.
Cook the turkey in the oven at 200°C (392°F) for 30 minutes.
Quarter the onions. Cut the carrot, celeriac, and leek into small pieces. Heat the rapeseed oil in a pan, fry the onions and chopped vegetables for 3 minutes, deglaze with white wine, and let the alcohol boil off for 5 minutes. Add the poultry stock to the pan.
Remove the turkey from the oven, brush with melted butter, and pour the vegetable stock mixture into the roasting pan. Return the turkey to the oven at 140°C (284°F) and cook for 3 hours, or until the thickest part of the thigh reaches 74°C (165°F) when measured with a meat thermometer.
Remove the turkey from the oven, place it on a serving tray, and return to the oven at 160°C (320°F) for 30 minutes.
Remove the turkey from the oven and let it rest for 20 minutes.
Carve the turkey and transfer to a serving platter with the apples removed from the cavity.
For the sauce: puree the gravy and soft vegetables from the roasting pan, season with salt and pepper, and serve with the carved turkey.