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Heat the oven to 180C/160C fan/gas 4. Peel and slice the potatoes as thinly as possible, ideally using a mandoline. Peel the onions and cut through the middle lengthways, then slice finely lengthways.
Layer the potatoes and onions in a circular pattern in a deep baking dish, about 30 x 20cm. Sprinkle the fennel seeds, lemon zest and a small pinch each of salt and pepper between the layers, until all the potatoes and onions have been used.
Pour over the stock and lemon juice. Dot the butter all over the top, then cover the dish loosely with foil. Cook in the oven for 45 mins, then remove the foil and cook for another 20-25 mins. It’s cooked when a knife goes easily through the middle. Rest for 10 mins before serving.