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Heat oven to 200C/180C fan/gas 6. Roll out the pastry and use to line a 23cm loose-bottomed tart tin. Prick the base with a fork, and line with baking parchment and baking beans. Bake blind for 12 mins, then remove the beans and parchment. Return to the oven for a further 5 mins until pale golden. Reduce oven to 180C/160C fan/gas 4.
Meanwhile, make the filling. Melt the butter in a large pan and, when foaming, add the leeks, cover and cook on a low heat for 20 mins until soft. Leave to cool.
Beat together the eggs, cream, milk, mustard, Caerphilly and some seasoning. Evenly cover the base of the pastry case with the leeks, then pour in the egg mixture. Bake for 25 mins.
To make the topping, combine all the ingredients. Remove the tart from the oven and sprinkle the topping evenly over. Return to the oven and bake for a further 15 mins until golden brown.