- Preheat oven to 240⁰C.
- Grease a 10cm round 1 cup-capacity cast iron baking dish with half the butter.
- Slice potato thinly with a mandolin.
- Layer potatoes, onion, salt, pepper, thyme, rosemary and butter evenly through each layer.
- Overfill by 2-3cm and press layers down firmly as you go.
- Finish with a knob of butter on top.
- Fill to ¾ full with stock.
- Bake in oven for ½ an hour with lid on, then another 40 minutes with lid off until golden.
- Quantities can be adjusted to go into a larger baking dish, just scale up recipe.