Tie the pork with string. On a high heat, add oil to a heavy based casserole pot. Season the meat well with salt and pepper. Sear the meat on each side until golden brown. Add the sliced onions and caramelise. Pour the port in, add the thyme and stock. Allow to come to the boil for 3 minutes.
Place the lid on the pot and cook on medium to low heat for 45 minutes, turning the meat three times during the cooking process.
Once cooked, remove the meat from the pot. Remove the string and thickly slice. Reduce the sauce for 5 minutes and finally pour over the sliced pork.