Coarsely crush the mustard seeds in the mortar. Mix mustard seeds and maple syrup, season paste with salt.
Wash the meat, dab dry and rub thickly all around with mustard paste. Roast in a roaster in a preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for approx. 2 3⁄4 hours.
Clean, wash and cut the leek into pieces. Wash the thyme and shake dry. Peel and wash the potatoes and cut them into halves or quarters depending on their size. Wash and quarter apples, remove core and halve apple pieces. Mix leek, apples, thyme and potatoes, season with salt and pepper. Arrange the vegetables around the roast about 45 minutes before the end of the frying time. 1⁄4 Pour on l water and cook everything to the end.
Take the finished roast out of the oven and let it rest for about 5 minutes. Cut into slices and arrange with potatoes and leek vegetables.