First, make the custard filling, which is really crème pattisserie. Beat 3 egg yolks and 85 g caster sugar until pale and creamy, then whisk in 1 Tbs plain flour
and 1½ Tbs cornflour. Add the grated zest of an orange to 250 ml milk and heat in a pan until almost boiling, then whisk this into the eggs. Pour it all back into
the pan, bring back to a boil and keep stirring until thick. Leave to cool completely. Now make the filo tart shell in a 23cm diameter loose-bottomed flan tin.
Lay a sheet of filo pastry on a worktop and brush one half with melted butter. Fold over, brush the top with butter and place into the flan tin, folding the overhanging
edges over itself to form the sides of the tart shell. Repeat with the next sheet of filo pastry, placing this at right-angles to the first. Butter and fold a third sheet
and place this into the tin at 45° to the last one, and again with the next sheet, placing this at 90° to the last, thus building up the tart evenly, and building the sides.
You can leave it at this stage, or go for another 4 layers, if you wish. Brush melted butter over the edges of the pastry and bake blind for 10 minutes in a 200°C oven,
then uncover and continue baking for another 5 minutes, until golden and crisp. Leave to cool. Now beat the cooled custard with an electric whisk until smooth and fluffy
and spoon into the pastry case. Chill for half an hour. Cut the skin and pith from 3 oranges, slice into thin rounds and arrange these in circles over the custard. Finally,
make some caramel, by melting 50 g caster sugar in a non-stick frying pan over a low heat, swirling from time to time until the caramel is dark gold, then drizzle this
over the oranges, taking care as it might spatter. Let it set for a few minutes, then serve straight away.