Grease a Swiss roll tin (jelly roll pan) and preheat the oven to 190C/375°F If using frozen raspberries, allow to defrost and drain.
Mix the flour and salt and roughly rub in the butter.
Stir the oats and sugar into the mixture and use your hands to combine (it will be a thick doughy mixture).
Press half of the oat mixture into the prepared tin. Sprinkle over the raspberries, squashing slightly if using fresh raspberries, to release the juice. Cover with the remaining oat mixture then press down firmly.
Bake for 20-25 minutes until golden brown. Leave to cool for 10 minutes then cut into squares.