Add the oats and milk to a small bowl, stir and then leave to soak for 5 minutes. Stir the mashed banana and raspberries through until combined.
Line a 19cm by 13cm microwaveable dish with baking paper and pour in the mixture, spreading out so it's even. Microwave on high for 3 minutes until firm. Leave to cool, then cut into fingers and serve with the yogurt to dip into.