Preheat the oven to 180C/350F/Gas 4.
First, make the biscuits. Mix together the butter, icing sugar and flour until smooth. Slowly add the egg whites, stirring, until they are completely incorporated, then fold in the currants. The dough will be quite wet. Place in a small bowl and chill for at least one hour.
Line a baking tray with greaseproof paper, and spread the mixture out. Chill in the fridge for 30 minutes.
Bake the biscuits for 25-30 minutes or until golden brown. While soft cut into slices with a sharp knife. Leave to cool, then keep in an airtight tin.
For the pastry, rub together the flour, salt, lemon zest and butter until the mixture resembles breadcrumbs. Add the sugar, then beat together the egg yolk and whole egg and slowly add these, mixing until the pastry forms a ball. Wrap tightly in cling film and refrigerate for two hours.
Turn the oven down to 170C/325F/Gas 3.
Roll out the pastry on a lightly floured surface to 2mm/one eighth-in thickness. Use to line an 18cm/7in flan ring placed on a baking sheet. Line with greaseproof paper and fill with baking beans, then bake blind for about 10 minutes or until the pastry is starting to turn golden brown. Remove the paper and beans, and allow to cool.
Turn the oven down to 130C/250F/Gas1.
For the filling, whisk together the yolks and sugar. Add the cream and mix well. Pass the mixture through a fine sieve into a saucepan and heat to blood temperature.
Fill the pastry case with the custard, until 5mm/¼in from the top. Carefully place in the middle of the oven and bake for 30-40 minutes or until the custard appears set but not too firm. Remove from the oven and cover the surface liberally with grated nutmeg. Allow to cool to room temperature.
Before serving, warm the biscuits through in the oven for 5 minutes. Cut the tart with a sharp knife and serve with the biscuits.