Melt the butter in a small pan over a low heat, skimming off any white liquid that rises to the top, then brush a generous layer of the melted butter over the base and side of a 20-22cm ovenproof frying pan.

Peel the potatoes and cut into 2mm-thick slices (use a mandoline if you have one). Put the slices in a large bowl of cold water as you go. When all the potatoes have been sliced, drain, cover with fresh water, and repeat once more to wash off the starch. Drain again and pat dry well using a clean tea towel.

Heat the oven to 200C/180C fan/ gas 6. Arrange a layer of potato slices in a neat, concentric circle in the base of the frying pan. Brush over some butter and season with sea salt. Repeat this until all the potatoes have been used, overlapping the slices as you layer them. Cook over a low heat for 10 mins to brown the potatoes on the base. Remove from the heat.

Meanwhile, cut out a 28cm circle of baking parchment. Fill an ovenproof pie dish or baking tin (it should be just wide enough to sit inside the top of the frying pan) with baking beans. Brush one side of the parchment disc with butter, and lay it butter-side down over the potatoes in the pan. Put the dish or tin on top to weigh down the potato layers and press them together, and bake in the oven for 25 mins. Remove the dish or tin and parchment, and bake for another 20 mins until the potatoes are soft and a skewer can be easily pushed through. Turn out onto a plate and slice into wedges to serve. Can be cooked a day ahead, cooled and kept chilled. Reheat in its pan in the oven, covered in parchment and foil, for 20 mins or until hot through.