Mix the mustard with all the other ingredients up to and including the pepper. Cover and set aside half of the mixture. Mix the garlic with the rest of the marinade. Coat the pork thoroughly with the mixture, cover and leave to marinate in the fridge for approx. 4 hrs. Remove the meat from the fridge approx. 1 hr. before cooking.
Peel the onions, pierce several times with a sharp knife. Mix the onions and tomatoes with the reserved marinade, season with salt.
To roast: Heat a baking tray in the lower half of an oven preheated to 220°C. Wipe the marinade off the meat, salt the meat. Place the meat fat-side down on the hot baking tray, brown for approx. 10 mins. on each side. Turn the oven down to 180 °C and cook for a further 15 mins. Mix the wine and water. Pour in half, add the onions, continue to cook for approx. 30 mins. Pour in the remaining liquid, add the tomatoes, cook for a further 30 mins. Remove the meat, cover and leave to rest for approx. 10 mins. Pour the wine over the vegetables on the baking tray and warm through in the switched-off oven.