- 25g butter, plus extra for greasing
- 1kg even-sized floury potatoes, such as King Edward or Maris Piper
- 1 onion, thinly sliced
- 284ml carton double cream
- 75ml milk
- Preheat the oven to 190°C/fan170°C/gas 5. Grease a large, shallow baking dish with butter. Slice the potatoes as thinly as you can, either using a mandolin, or with a very sharp knife.
- Layer up evenly in the baking dish, along with the onion slices, seasoning as you go. Pour over the cream and milk, dot over the butter and cover the dish with foil.
- Place on a baking sheet and bake for 1 hour.
- Discard the foil and bake for a further 15-20 minutes, or until the potatoes are cooked through and golden.