For the almond cream, heat 1 tablespoon of butter in a frying pan. Lightly roast ground almonds in it while stirring. Allow to cool slightly and mix with 100 g icing sugar. Cream 100 g butter with the whisk of the mixer. First stir in the almond mixture, then 1 egg. Finally, briefly stir in the flour. Chill the almond cream for about 30 minutes.
For the base, place dough plates next to each other and let them thaw. Place the dough sheets on top of each other and roll out into a rectangle (approx. 24 x 48 cm) on a floured work surface. Spread almond cream on top, leaving a rim of approx. 2 cm all around. First fold over the short sides, then the long sides. Put the dough strips on baking paper and refrigerate for about 1 hour (very important, otherwise the almond cream will melt!).
Wash the nectarines and cut the flesh into slices from the stone. Place the dough on a baking tray with baking paper. Whisk 1 egg yolk with milk and brush the dough with it. Cut the dough with a knife at a slight angle all around (do not cut through!). Place nectarine slices on the almond cream. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for 35-40 minutes. Let them cool down.
For the brittle, heat 1 tablespoon of butter in a frying pan. Roast the almond slivers in it. Sprinkle sugar over it and let it caramelize. Spread the brittle on baking paper and let it cool down. Dust nectarine strips with 1 tbsp. icing sugar and sprinkle with brittle.