First make the cinnamon biscuits. Sift the flours with the salt and cinnamon. Cream the butter with the sugar until pale and fluffy. Beat in the egg yolk. Add the flour mixture and bring together with your hand, adding a few drops of water if necessary to help bind to a smooth dough. Wrap and chill for 20-30 minutes.
Divide the dough in half and roll out each half thinly (to about
To make the custard, pour the milk and cream into a heavy-based saucepan and add the cinnamon sticks and ground cinnamon. Heat until bubbles start to form round the edge of the pan, then remove from the heat and set aside to infuse for 20 minutes. Meanwhile, put the egg yolks and sugar in a heatproof bowl set over a pan of hot water and whisk until very thick and a pale lemon colour. Strain the hot milk mixture into the yolks, stirring to mix. Return the mixture to the saucepan. Set over a low heat and cook, stirring constantly, for about 10 minutes or until the custard thickens. You should be able to run your finger across the custard on the back of the spoon and leave a clear trail. Keep the custard hot.
Peel, core and roughly dice the apples, then sprinkle with the lemon juice and toss to coat. Melt the butter in a wide pan until foaming, then add the apples with the lemon juice and the cinnamon. Cook gently, stirring occasionally, for about 15 minutes. Add sugar to sweeten to taste and stir until dissolved. Add the sultanas and walnuts and warm through for a few minutes.
Pour the custard into soup plates and spoon the apple compote into the middle. Serve hot with the cinnamon biscuits.