1. To make the tart dough, cream the butter in the mixer until light and fluffy. Gradually add the sugar, icing sugar, walnuts and salt. Stir in the egg, carefully add the flour and work everything together to make a smooth dough. Form into a ball, wrap in cling film and leave to rest in the fridge overnight.
2. Roll out the dough evenly on a lightly floured surface and place into a 28 cm diameter tart dish. Press down the edges using your fingers and cut off any excess dough using a knife.
3. Wash and halve the apples. Cut the apples into thin slices.
4. Preheat the oven to 190 degrees top/bottom heat (fan 175 degrees). Arrange the apple slices on the dough in rings.
5. To make the glaze, whisk the sugar, eggs, cream and lemon zest together well in a bowl. Spread evenly over the apple slices. Place into the oven and bake for around 35-40 minutes.