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Heat oven to 190C/170C fan/gas 5. Put the flour in a food processor with the butter and sugar, and mix until it forms fine breadcrumbs. Add the egg yolk and 1-2 tbsp cold water, and pulse to make a firm dough.
Have ready a 24cm tart tin. Briefly knead the pastry on a lightly floured surface, then roll out to a round about 5cm larger than your tin. Lift onto the tin with the help of your rolling pin, then press into the corners using your finger. Do not trim the pastry. Fill the pastry case with a round of baking parchment and baking beans. Chill for 10 mins. Blindbake for 10 mins, remove the paper and beans, then bake for 5 mins more.
To make the filling, put the sugar in a large pan with 3 tbsp cold water. Heat gently, stirring to dissolve the sugar. When the sugar is completely dissolved, increase heat and bubble until the syrup has turned a rich caramel colour. Remove from the heat and stir in the butter until it has dissolved, then stir in the cream. Return to the heat and boil hard, stirring until the sauce is thick enough to leave a gap on the base of the pan when you draw your spoon across it. Stir in the walnuts.
Fill the pastry case with the nut mixture, levelling it with a fork. Return to the oven for 8-10 mins until the filling is bubbling. Cool for 10 mins before removing carefully from the tin. Serve warm or cold.
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