Prepare flaxseed egg by combining flaxseed with water in small bowl. Set aside for 15 minutes.
Preheat oven to 180c
Line loaf tin (21 cm x11 cm) with baking paper.
Sift flour into large mixing bowl. Add sugar, butter, milk, banana, vanilla and flaxseed egg. Fold in gently with spatula until just combined. Transfer into lined tin. Scatter oats or muesli over batter and place in preheated oven for 45 minutes or until inserted knife comes out clean.
Allow to cool on wire rack for 10 minutes before serving.