Put all the dry ingredients into a bowl: self-rising flour, baking soda, cinnamon, sugar, oats, walnuts and salt. Mix together.
Take a separate bowl for the wet ingredients. Add the banana first and blend until smooth. Add the milk, egg, sunflower oil and vanilla extract and mix until smooth. Pour this mixture into the dry ingredients and mix with a spatula until you have an even batter.
Divide the dough between the tins, filling them about ¾ full. You can fill them more, but remember that these are big muffins. I used silicone moulds so they come out easily.
Bake the oatmeal muffins in the oven for 18-20 minutes at 200°C/400˚F (conventional oven).
How to store
Keeps fresh in an airtight container outside the fridge 3-4 days. In the freezer, they keep for up to 3 months.