Preheat oven to 350 degrees F (175 degrees C).
Combine vegetable oil, mustard, caraway seeds, garlic powder, salt, and pepper to form a paste. Rub over the pork roast, and allow to season about 30 minutes.
Arrange onions in the bottom of a roasting pan. Pour in beer. Place roast, fat side down, on top of onions. Cover 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 160°
Remove from heat, let cool about 20 minutes. Reserve pan juices.
Bring the reserved pan juices to a boil. Mix in butter and cornstarch to thicken, reduce heat, and simmer 5 to 10 minutes to make gravy.
Serve the pork thinly sliced drizzled with gravy.