Place the biscuits and Walnuts in a food processor and blitz to give fine breadcrumbs, stir in the butter and press into the base and sides of a 23cm fluted flan tin. Bake for 10 minutes and allow to cool.
Meanwhile, whisk the egg yolks until pale and thick, then add the condensed milk, whisk for 2-3 minutes then add the lime zest and juice and whisk for a further 2-3 minutes, stir in ¾ of the Walnuts. Pour into the biscuit base, sprinkle with remaining Walnuts and bake for 20 minutes. Allow to cool before chilling. Serve with lime zest sprinkled on top.
