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