1. Preheat oven to 400 degrees Fahrenheit.
2. Combine ketchup, duck sauce, chicken soup, and garlic cubes until smooth.
3. Place the roast in a pan and pour the sauce over it. Spread the sliced onions on top of the sauce. Bake uncovered for one hour, then lower the temperature to 350 degrees Fahrenheit, cover the meat tightly, and bake for an additional three hours.