Preheat oven to 180°C. Grease a 10cm x 20cm (base measurement) loaf pan and line the base and sides with baking paper.
Use an electric mixer to whisk the egg, egg white and maple syrup in a bowl for 5 minutes or until the egg mixture is thick and firm peaks form.
Fold in half the flour and half the yoghurt until just combined. Add the bicarbonate of soda, lemon rind, lemon juice, remaining flour, remaining yoghurt and half the rhubarb. Fold until just combined.
Spoon the mixture into the prepared pan and smooth the surface. Top with the remaining rhubarb, gently pressing into the surface. Bake for 40 minutes or until a skewer inserted in the centre comes out clean. Transfer to a wire rack and set aside for 10 minutes to cool slightly. Cut into slices and serve warm.