Preheat the oven to 180°C / 360°F and line a baking tray with parchment.
Blend oat flakes and walnuts into a fine crumb and transfer to a bowl.
Add vanilla sugar, sesame seeds and pumpkin seeds and mix well.
Mash the banana with a little lemon juice to a smooth puree.
Combine the banana puree with the dry mixture and stir until coated.
Add 100 ml water and mix until the dough is thick and sticky, adding a little more water if needed.
Shape the dough into small balls and place them on the prepared tray.
Flatten each ball gently into a round cookie of even thickness.
Bake for about 30 minutes until lightly golden and set at the edges.
Cool briefly on the tray, then transfer to a rack and allow to cool completely.