Preheat oven to 160°C.
In a wide saucepan, over medium heat, melt the butter , sugar with the grated apple, and rhubarb.
Cook the mixture over low heat for 5-8 minutes until the apple are fully cooked and broken down into a puree. Remove from the heat and set aside to cool for 10 minutes.
Roll the puff pastry and line the base of a tart dish or tin. Prepare the lattice topping too by cutting a circle the size of the diameter of the dish and using a lattice roller or a knife create the indentation on the pastry.
Ensure the pastry is dented fully using a knife to check. Gently pull the pastry to create the lattice ‘accordion’ pattern.
Place the lattice in the fridge to help settle and to be easier to handle for 5 minutes or so.
Add the apple filling in the tart base and cover the top with the cooled lattice pastry, again ensuring the lattice indentation are all visible and stretched as equally as possible.
Brush the pastry with some egg wash and sprinkle with icing sugar.
Transfer in the oven and bake for 30-40 minutes until the pastry is fully cooked.
Remove from the oven and set to cool for 10 minutes.
Sprinkle with extra icing sugar and serve warm or at room temperature with some whipped cream.