Preheat the oven to 185 C (170 C fan). Line the 12-whole muffin tin with paper cases (alternatively, use 6-hole jumbo muffin tin) and set aside. Reserve about 2 Tbsp of brown sugar and 2 Tbsp of oats and set aside. In a large bowl, mix together the flour, baking soda, remaining sugar, remaining oats, cinnamon and salt.
In a separate bowl, beat together mashed bananas, eggs, buttermilk and olive oil until well combined. Pour this mixture into the bowl with dry ingredients and using a wooden spoon or spatula, stir all the ingredients together until JUST combined. Few lumps and streaks of flour remaining are absolutely fine, don't be tempted to overmix the batter. Reserve a handful of blueberries, then add the remaining blueberries into the batter and give it a final fold to mix them in.
Divide the batter between the muffin cases. Fill them up all the way up to the top, top each muffin with few extra blueberries, then sprinkle the remaining sugar and oats over the top of each muffin and bake for 18 - 20 minutes until they muffins have risen and their tops are golden brown. Remove from the oven and allow them to cool for 5 minutes, then remove from the baking tin and transfer them into a cooling rack.
To stop the muffins sticking to the paper cases, grease the paper cases lightly with some olive oil (using a pastry brush) or melted butter.
Self-rising flour- if no self-rising flour is available, you can use plain flour. General rule is to use 2 tsp of baking powder for every 150 g of flour, but in this recipe, 3 tsp of baking powder will be more than enough with 320 g of flour.
Store the muffins in the airtight container for up to 3 days.
To freeze, wrap each muffin tightly in plastic or kitchen foil, then transfer to the freezer-friendly container or a freezer bag and freeze for up to a month.