Prepare tin and oven: Line and grease a 20–22 cm cake tin, preheat oven to 180∘C.
Mix dry ingredients: Combine oats, flour, baking powder, salt and cinnamon in a bowl.
Add apple and zest: Grate the apple and lemon zest, keeping all juices, ready to add.
Whisk wet base: Beat yoghurt, eggs, honey or maple syrup and vanilla until smooth.
Combine batter: Fold wet mixture into dry ingredients, then add grated apple and zest.
Fold in blueberries: Gently mix in blueberries without crushing them.
Fill tin: Pour batter into the prepared tin and level the surface.
Bake: Bake for 30–35 minutes until golden and a skewer comes out mostly clean.
Cool: Rest in the tin briefly, then cool completely on a wire rack.
Serve: Slice into eight portions and enjoy plain or with yoghurt and extra berries.