In a bowl, form a paste using the vegetable oil, mustard, caraway seeds, garlic granules, salt and pepper. Rub over the pork and let sit about 30 minutes.
Preheat oven to 180 C / Gas 4.
Arrange the onions in the bottom of a large roasting tin. Pour in the beer. Place the roast, fat side down, on top of the onions. Cover the tin with foil.
Roast 1 hour in the preheated oven. Remove foil, turn roast and score the fat. Continue roasting 2 1/2 hours or to a minimum internal temperature of 70 degrees C. Remove from heat, reserving tin juices and let sit about 20 minutes before slicing thinly.
In a saucepan, bring the reserved tin juices to the boil. Mix in the butter and cornflour to thicken, reduce heat and simmer 5 to 10 minutes. Serve with the sliced pork.