Melt the butter in a small saucepan over a medium heat. Arrange the potato slices on top of the hotpot, overlapping the edges slightly. Brush the potato slices all over with the melted butter.
Preheat the oven to 200C/180C Fan/Gas 6. Heat a large saucepan over a medium heat. Add the oil and, once hot, add the onion and a pinch of salt. Cook the onions for 4-5 minutes, or until softened ...