Combine the flour, sugar, and salt ( use only if your butter is unsalted) in a large bowl.
Rub the chilled butter cubes in the mixture, using your fingertips. When it resembles coarse breadcrumbs, add iced cold water, 1 Tbsp at a time, just enough to bind the dough. It can be a bit scraggy at this point but don’t worry, it will come together. Using the palm of your hand, push the dough away from you 4-5 times until its smooth.
Roll into a ball and flatten it into a disk. Wrap in clingwrap and refrigerate until ready to use. This should rest for at least an hour.
When you’re ready to bake, line a baking tray with parchment paper or silpat. Preheat the oven to 180C.
Filling
Toss the apple slices with lemon juice, sugar, cinnamon powder, and raw sugar.
Cut the marzipan into small pieces.
Unwrap the dough and flour your work surface well. Roll the dough out carefully into a rough 8" round. Don’t worry if it breaks or you don't have a perfect circle.
Scatter the marzipan pieces on the rolled out dough, leaving at least an inch of border. Arrange the apple slices on top and fold the edges of the dough over the fruit to partially cover them.
Sprinkle the almond flakes all over the apples and on top of the pastry too.
Bake at 180 C for 30-35 minutes until the pastry is light golden brown and the fruits are bubbly. Let it rest for at least 15 mins. Serve warm or at room temperature. Enjoy!