Preheat the oven to 190°C (375°F). Line a 12-hole muffin tin with paper muffin cases or grease and flour them well.
Place the bananas into a large bowl or jug and mash well.
Add the eggs, oil, and sour cream and mix until well combined. Put aside.
Sift the flour, baking powder, and mixed spice powder into another large bowl.
Add the oats, brown sugar, and chopped nuts to the flour and stir to combine. Then add the blueberries and stir to coat in flour.
Make a well in the center of the mixture and pour the wet ingredients into the dry.
Use a wooden spoon to stir the mixture until it just combines. The batter should be a little lumpy.
Spoon the mixture evenly into the prepared muffin tin holes.
Place into the oven and bake for about 15 - 20 minutes or until they have risen and cooked. They are cooked when you can insert a skewer into one of the muffins and it comes out clean.
Allow the muffins to stand in the tin for 5 minutes then transfer them to a wire rack. Serve, hot, warm, or at room temperature.