Trim any excess fat from the pork. Combine garlic, mustard seeds, mustard powder, Dijon mustard, salt and pepper in a bowl to form a paste. Evenly spread paste over pork; wrap and chill in the fridge overnight.
Preheat the oven to 180 C / Gas 4. Place pork in a roasting tin.
Roast pork in the preheated oven uncovered, about 1 hour or until internal temperature measured with a meat thermometer reaches 70 degrees C. Remove from oven, and let rest about 10 minutes before slicing to serve.
Meanwhile, combine wine, chicken stock and onion in a large saucepan. Bring to the boil and reduce liquid to 250ml, about 45 minutes. When pork is removed from the oven, pour sauce into roasting tin. Bring to the boil, scraping up any browned bits. Serve sliced roast with sauce.