For the dough, mix the whole flour, oat flakes, salt and 3 tablespoons of the core mixture in a mixing bowl. Add 375 ml cold water and oil. Knead everything first with the dough hooks of the mixer, then with your hands to a firm dough.
Rinse two baking trays with cold water. Halve the dough. Press one half of the dough thinly with moistened hands on a baking tray (approx. 30 x 30 cm). Cut each dough sheet into approx. 15 pieces (each approx. 6 x 10 cm). Sprinkle the remaining seeds on top and press down slightly.
Bake one after the other in a preheated oven (electric oven: 230 °C/circulating air: 210 °C/gas: see manufacturer) for about 5 minutes. Turn down the oven temperature (electric oven: 200 °C/circulating air: 180 °C/gas: see manufacturer). Bake crisp bread for 20-25 minutes until crispy. Remove from the baking tray immediately and allow to cool on a cake rack.