1. Thinly slice potatoes and add to saucepan of water. Boil until softened.
2. Cube the onion and place in saucepan with potatoes. Add the milk, cover saucepan and bring to the boil.
3. Simmer for 3 mins then drain. Butter an ovenproof dish and layer half of the potatoes (so they overlap) in it. Sprinkle half of the onions on top then season with salt & pepper.Sprinkle cheese on top.
4. Layer the remaining potato on top. Add rest of the onion and cheese. Season again with salt & black pepper.
5. Bake at 190c for 35mins or until crispy and golden.