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In a large pan, heat a knob of the butter with a drizzle of oil. Once the butter is foaming, add the onions, leek and some seasoning and cook for 15 mins until the onions are soft and sticky, stirring occasionally. Butter a 22cm cake tin or baking dish (alternatively, you can assemble the cake in the pan you’re already using if it’s ovenproof).
In a separate pan, melt the remaining butter. Toss the sliced potatoes with the melted butter and plenty of seasoning. Heat oven to 200C/180C fan/gas 6.
Arrange the potato slices over the base of the dish in a spiral or in concentric circles. Top with some of the onion mix, sprinkle over a layer of cheese and continue layering, finishing with a final round of potato slices. Press down firmly with your hands to compact the layers. Bake for 45-50 mins until the potatoes are golden brown and tender; cover with foil if the cake starts to take on too much colour. Turn out onto a plate to serve.