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Put the flour, butter and salt into the bowl of a food processor, then pulse until the mix looks like fine crumbs. Splash in 4 tbsp cold water, then pulse to a dough. Turn onto a lightly floured surface, shape into a smooth disc, then wrap and chill for at least 10 mins. Heat oven to 200C/fan 180C/gas 6.
Roll pastry out on a floured surface until large enough to line a 23cm loose-bottomed tart tin. Line the tin with the pastry, leaving any excess pastry overhanging. Line with baking paper, then fill with baking beans. Bake on a baking sheet for 15 mins. Take out the paper and beans, then bake for 10 mins more until pale golden and cooked. Use a serrated knife to trim the pastry level with the tin – resting the knife flat on the edge of the tin will help achieve an even finish.
While the pastry cooks, soften the onions in the oil over a medium heat for 10 mins until golden. Beat the eggs and cream together, then season to taste. Crumble up the hard cheeses and chop or pull the creamy cheese into small pieces. Scatter the cheese into the pastry case, add the onions, then pour in the egg mix. Turn the oven down to 160C/fan 140C/gas 3 and bake for 40 mins until set and lightly golden.