Preheat the oven to 220 C / Gas 7. Grease a 12-hole muffin tin.
Blend flour, bicarbonate of soda, salt and baking powder together in a bowl. Make a well in the centre; add eggs, maple syrup, applesauce and coconut oil. Stir by hand until just combined. Add bananas, blueberries and walnuts. Mix until just incorporated. Divide mixture evenly into the prepared muffin tin.
Bake in the preheated oven, 6 to 7 minutes. Reduce heat to 180 C / Gas 4. Continue baking until a skewer inserted into the centre of a muffin comes out clean, about 15 minutes more.
Cool muffins in tin for 5 minutes before transferring to a cooling rack to cool completely.