In a bowl, whisk the egg yolk, cream, zest from one orange and caster sugar together.
Butter a small gratin dish, add the chopped walnut sourdough and pour over the creamy mix. Leave to soak for three minutes.
Place the gratin dish in the microwave and cook for five minutes on high.
Rest for one minute and then cook for a further 1-2 minutes if the mix has not set.
To serve, slice the remaining orange and place the orange slices on top of the gratin. Dust with icing sugar.
