Heat the oven to 180°C. Line a small (about20x20 cm) baking tin.
Blend the bananas and apple in a food processor. Add the spices and half of the oats, then whiz until you get a rough mixture. Add the walnut and the remaining oats and blitz again for a few seconds. You should get a thick, textured batter, not a hard dough.
Transfer the mixture to the tin, spreading it out evenly.
Bake in the middle of the oven for 25 minutes.
Let it to cool in the tin for 10 minutes, then transfer it to a wire rack and let it cool completely.
Cut it into bars or squares.