Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Finely crush mustard seeds in a mortar. Mix with maple syrup and oil. Season with approx. 1 tsp. salt.
Wash the meat, dab dry and rub all around with the mustard paste. Place roast on a fat pan (deep baking tray) or in a large roasting pan. Bake in the oven for 2 1/2-3 hours.
If necessary, cover the roast carefully with aluminium foil after about 2 hours frying time, but the foil should not stick to the paste.
Clean the leek and cut into pieces. Peel and wash the potatoes and cut them into halves or quarters depending on their size. Wash apples, quarter them, remove seeds and cut them in half crosswise. Wash thyme, shake dry.
Mix the prepared ingredients and after approx. 1 hour frying time, spread around the roast. Season with salt and pepper. Roast for about 10 minutes, turning from time to time. Dissolve the stock in approx. 1⁄8 l hot water and pour on the apple juice.
Finish frying everything.
Remove the finished roast and let it rest for about 5 minutes. Cut the roast open. Season vegetables and gravy with salt and pepper. Arrange everything.