For the peach gratin with meringue topping, first crumble the biscuits. Cut the marzipan into small cubes and mix well with the crumbled biscuits. Skin the peaches, cut them in half and remove the stones. Cut the flesh into finger-width slices. Mix everything together. Preheat oven to 125 °C.
Pour fruit mixture into a gratin dish (of approx. 25 x 25 cm for 2 kg peaches). Beat egg whites until stiff, gradually adding sugar. Continue beating until you have a glossy, firm mixture. Using a wide nozzle, pipe this egg white mixture across the peach mixture.
Sprinkle the flaked almonds over the top and bake in the oven for 90 minutes, until the meringue shows light browned tips. If the egg whites brown too much, cover the peach gratin with meringue with parchment paper.