Preheat oven to 240°C. Grease a 10 cm 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.