In an overproof saucepan, heat oil. Brown the pork on all sides.
Let drain the pineapples and keep the juice.
In a bowl, mix pineapple juice and water. Incorporate the molasses, mustard, salt and pepper. Pour on the pork. Cover. Cook in oven for 1 hour. Baste with the cooking juice each 15 minutes. Remove the lid for the last 15 minutes of cooking.
Put the pork roast in a serving dish. Keep it warm.
In a bowl, mix the cornstarch with the chicken stock. Set aside.
Skim the cooking juice. Add vinegar and the cornstarch with chicken stock. Bring to boil stirring constantly. Add the pineapple chunks. Adjust seasoning. Pour on the slices of pork.