For the strudel:
Preheat oven to 220ºC / 425ºF / gas mark 7.
Fry the apple, caster sugar, two thirds of the butter and cinnamon over a moderate heat until caramelised. Add the banana and allow to cool.
Melt the remaining butter and brush it on the filo pastry. Dust the filo with icing sugar and cut in half. Lay one sheet over the other, place the filling in the middle and roll up. Cook in oven for 6-8 minutes.
For the custard:
Bring the milk to simmering point slowly over a low heat.
Whisk the egg yolk, sugar and cornflour together in a bowl until well blended. Pour the hot milk on to the eggs and sugar, whisking continually. Return to the pan, add vanilla essence and over a low heat gently stir until thickened.
Slice the strudel into two and serve with the custard.
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