For the pastry, mix the flour and icing sugar together in a bowl. Add the butter and rub it in lightly with your fingertips until the mixture looks like fine breadcrumbs. Alternatively, do this in a food processor or a mixer and then transfer to a bowl.
Mix the egg with the lemon juice and a tablespoon of water. Make a well in the centre of the flour mixture and pour in the egg mix. Using a table knife, work the liquid into the flour to bring the pastry together. If it seems too dry, add a splash more water. When the dough begins to stick together, use your hands to gently knead it into a ball. Wrap in cling film and rest in the fridge for at least 15 minutes.
Preheat the oven to 200C/400F/Gas 6.
Roll out the pastry on a lightly floured surface to about a 3mm thickness (about the thickness of a pound coin) and use it to line the tart tin, leaving excess pastry hanging over the edge. Keep a little uncooked pastry back in case you need to patch any cracks later.
Line the pastry case with baking parchment or foil, then fill with baking beans, or uncooked rice or lentils. Bake blind for 15 minutes, then remove the parchment and baking beans and return the pastry to the oven for about eight minutes or until the pastry looks dry and faintly coloured.
Use a small, sharp knife to trim away the excess pastry from the edge. Use a tiny bit of the reserved raw pastry to patch any cracks or holes if necessary.
Turn the oven down to 180C/350F/Gas 4.
To make the filling, beat the sugar and curd cheese together until smooth, then beat in the eggs and egg yolks, lemon zest, rosewater and melted butter. Stir in the currants.
Pour the filling into the pastry case and grate a little nutmeg over the surface. Bake for about 20 minutes, until the filling is just set. Leave to cool completely in the tin before slicing.