Preheat the oven to 180°C.
Mix butter, icing sugar, and flour until smooth, then stir in egg whites and fold in currants.
Spread the dough onto a tray lined with greaseproof paper and chill for 90 minutes total.
Bake biscuits for 25-30 minutes until golden, then slice while soft and cool.
Rub flour, salt, lemon zest, and butter together, then mix in sugar and eggs to form a ball.
Wrap the pastry in cling film and refrigerate for two hours.
Lower oven to 170°C and roll pastry to 2mm thickness.
Line an 18cm flan ring with pastry and bake blind with beans for 10 minutes.
Lower oven to 130°C and whisk egg yolks, sugar, and cream together.
Sieve the custard into a pan and heat to blood temperature.
Fill the pastry case with custard and bake for 30-40 minutes until set.
Grate nutmeg over the surface and allow to cool to room temperature.
Warm biscuits for 5 minutes at 130°C before serving with the sliced tart.