Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). Wash meat and place in a 9×13-inch baking pan. Season lightly with onion powder, garlic powder, salt, and paprika. Crush garlic and smear all over the roast. Pour sherry cooking wine and soy sauce over the roast. Cover tightly with aluminum foil and bake for two and a half to three hours.

Remove roast from oven. Preheat oven to broil. Remove foil from roast and cut shallow slits, one inch apart, across the top of the roast. Combine brown sugar and mustard in a small bowl and spread over the roast. Broil three to five minutes, until brown sugar mixture begins to bubble and caramelize. Remove from oven. Allow to cool and then slice.