Preheat the oven at 180°C.
In a bowl add all the ingredients except the potatoes and butter and mix well. Take the butter and using your hands grease a small oven dish. Place the potatoes in the dish. Sprinkle the onion on top. Then pour the liquid mix over and add more cheese on top. Bake in the oven for 50 minutes or until the potatoes are well cooked and golden brown on top.