Preheat the oven to fan 170C/ conventional 190C/gas 5. Butter a shallow 23cm square tin. Tip the flour, oats and butter into a mixing bowl and use your fingers to work the mixture together to make coarse crumbs. Mix in the sugar, lemon zest and three quarters of the pine nuts using your hands, then press the mixture together well so it forms large sticky clumps.
Drop about two thirds of the oat mixture into the base of the tin, spread it out and press down very lightly – don’t pack it too firmly. Scatter the raspberries on top, sprinkle the rest of the oat mixture over, then the rest of the pine nuts and press everything down lightly.
Bake for 35-40 minutes until pale golden on top. Cut into 12 bars with a sharp knife while still warm, then leave to cool in the tin before removing. Will keep for 2-3 days.