Place the pork in a pan. In a bowl, combine all the marinade ingredients and mix well. Pour the marinade over the pork. Turn the meat to ensure even coating with the marinade. Cover the pan and refrigerate for 8 hours or overnight.
Peel and wash the potatoes, then cut them into cubes. Place them in a pot, cover all with tap water, and bring to a boil. Cook until they soften, about 20 minutes. Drain and transfer to a bowl. Add butter, salt, white pepper, and mash using a potato masher. Add the grated cheddar cheese to the mashed potatoes and mix with a whisk.
Meanwhile, remove the pan from the refrigerator to bring the meat to room temperature. Preheat the oven to 180°C. Bake the meat for 20 minutes, then turn it over and continue baking for an additional 20-30 minutes.
Serve the pork with the mashed potatoes. Drizzle with the sauce from the pan and garnish with finely chopped fresh green onions.