Preheat the oven to 175°C, conventional heat. Line a loaf tin with parchment paper.
Mash the banana and mix with the Greek yogurt, honey or maple syrup, eggs, vanilla extract, and cinnamon.
Add the oat flour, almond flour, baking powder, and salt. Stir until just combined.
Gently fold in the apple slices, separating them so they are evenly distributed.
Pour the batter into the prepared tin and smooth the top.
Bake for approximately 40–55 minutes, checking from 35–40 minutes. The centre should be set, and a skewer should come out with moist crumbs but no wet batter. Cover loosely with foil if the top browns too quickly.
Leave to cool in the tin for about 15 minutes, then carefully transfer to a wire rack to cool completely.