Sift the flour into a bowl, make a well in the centre, then add the butter, sugar, egg yolks, salt and water. Rub together lightly until mixture forms a soft dough. Wrap in cling film. Chill for 30 minutes.
Roll out the pastry on a lightly floured surface until large enough to line a 23 cm quiche tin. Line the pastry shell with baking paper and dried beans.
Bake blind for 15 minutes in a preheated oven at 200 °C.
Remove the baking paper and beans, then bake for a further 10 minutes.
Whisk the egg yolks until creamy then gently beat in the condensed milk, lime or lemon juice, rind and food colouring (optional), until thickened.
Whisk the egg whites in a separate bowl until they start to form soft peaks, then whisk in the castor sugar until stiff. Gently fold into the lime mixture.
Pour the filling into the prebaked pastry case. Reduce the oven temperature to 160 °C and bake the pie for 20 to 25 minutes, or until set and just starting to brown. Remove from the oven, cool, then transfer to the fridge until ready to serve.
Spoon the whipped cream around the edges. Halve the lime or lemon slices, place on top of the cream and scatter with the reserved grated lime or lemon rind.