Rub the neck with salt and pepper and fry all around in oil. Pour half of the stock, roast the roast in the preheated oven (electric: 175 °C/ gas: level 2) for one hour.
Pour in the remaining stock little by little. In the meantime wash the thyme. Remove the leaves from half of the thyme. Peel the celery and carrots. Cut the celery into cubes of about 1.5 by 1.5 cm.
Cut carrots diagonally into wide slices. Fry the vegetables and thyme leaves in the remaining fat. Remove 100 millilitres of the roast stock, add to the vegetables and steam for 15 minutes over a mild heat.
Season to taste with salt and pepper. Keep the roast warm in the switched off oven. Pour the roast stock through a sieve and bring to the boil, add sauce thickener and bring to the boil again. Season the sauce with salt and pepper.
Cut the roast open and arrange with the vegetables, garnish with the remaining thyme. Serve with mashed potatoes and chives rolls.