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First, make the pastry. Tip the flour and butter into a bowl, then rub together with your fingertips until the mixture resembles coarse breadcrumbs. Mix in the egg, then bring everything together using your hands until just combined. Form into a disc. Will keep wrapped and chilled for up to two days or frozen for up to a month.
Roll the pastry out on a lightly floured surface into a disc that’s roughly 28cm in diameter. Use the rolling pin to lift it over a 23cm tart tin, then drape it over the tin. Use a small ball of pastry scraps to push the pastry into the base and up the side of the tin, leaving the excess to overhang. Chill in the fridge for 20 mins. The tin can be lined and chilled up to a day ahead.
Meanwhile, melt the butter in a pan over a low-medium heat and cook the onions for 45 mins, seasoning them generously with salt and stirring occasionally until they become sticky and golden. Pour in the cream, then add the nutmeg and stir. Bring to a simmer, then remove from the heat.
Heat the oven to 200C/180C fan/gas 6. Lightly prick the base of the tart all over with a fork, then line with a large circle of baking parchment and fill with baking beans, dried rice or pulses. Bake for 20 mins, then remove the beans and parchment, and bake for 10-15 mins more until light brown. Break off a little of the the overhanging pastry to check that it’s done – it should be cooked through and crumbly.
Meanwhile, stir the egg yolks into the onion and cream mixture, and stir in two-thirds of the cheese. Tip into the pastry case, then sprinkle over the rest of the cheese and bake for 20-25 mins, or until just set and golden. Leave to cool completely in the tin, trim the overhang using a serrated knife, then remove from the tin. Sprinkle with chives and more cheese, if you like. Any leftovers will keep chilled for up to two days. Reheat in a low oven before serving.