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Put the flour, butter and a good pinch of salt into a food processor, and blitz until the mix resembles breadcrumbs. Combine the yolk with 1½ tbsp very cold water, then add this to the mixture and blitz again until the pastry comes together into a ball (you may need to add a little more water).
Lightly dust a rolling pin and a large sheet of baking parchment with flour. Roll the pastry out on the parchment, and use it to line a 23cm loose-bottomed tart tin. Chill for 1 hr until firm. Heat the oven to 200C/180C fan/gas 6 with a baking sheet inside.
For the filling, heat the butter in a heavy-bottomed pan over a medium heat, add the leeks, stir, season and add 2 tbsp water. Cover, reduce the heat to low and cook for 15 mins until the leeks are soft. Uncover, turn up the heat and cook until any liquid has evaporated. Remove from the heat and leave to cool.
Prick the base of the pastry case all over with a fork, then line with baking parchment and baking beans. Bake on the hot baking sheet for 12 mins, remove the parchment and beans, then bake for 13 mins more. Leave to cool slightly, then spoon the leeks into the pastry case.
Mix together the double cream, crème fraîche, whole egg and egg yolks. Season well, then gently fold in the crabmeat. Carefully pour the filling over the layer of leeks, then scatter over the cheese.
Return the tart to the baking sheet in the oven and bake for 35-40 mins, or until the pastry is golden and the filling is just set in the middle (it will continue to cook as it cools). Leave to cool for 15 mins, then remove from the tin and serve.