Preheat the oven to Gas Mark 3, 170ºC, fan 150ºC. Arrange 8 muffin cases in a muffin tin and set aside.
In a large mixing bowl, whisk the butter, sugar and egg with an electric whisk. Sift in the flour and baking powder, then gradually add the milk, mixing continuously until light and fluffy.
In a separate bowl, mash the bananas with the lemon juice until smooth. Fold the banana mixture and raspberries into the batter, then divide the mixture between the muffin cases. Bake in the oven for 25 minutes, until golden. Remove from the oven and transfer the muffins to a wire rack, to cool.
See more Gluten-free recipes