Brown the duck legs skin-side down in a pan. Once the fat has rendered, place the legs meat-side down on paper towels. Remove the skin and shred the meat into large pieces. Sauté the duck meat in a hot skillet with the sliced onions until the onions are translucent. Drain everything in a colander.

Meanwhile, place the potatoes (with skins on) in cold water and bring to a boil. Cook until just tender but not overcooked. Let them cool, then peel.

Butter a baking dish. Spread the duck and onion mixture on the bottom, then top with the mashed potatoes. Dot with pieces of butter. Bake at 400°F (200°C) until the top is golden brown. Delicious served with a green salad!