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 350 degrees F (175 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, or to a minimum internal temperature of 145 degrees F (63 degrees C). 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 cornstarch to thicken, reduce heat, and simmer 5 to 10 minutes. Serve with the sliced pork.