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Heat oven to 160C/140C fan/gas 3. Grease and line the base of a 20cm square baking tin with baking parchment so that there is a slight overhang – this will make it easier to remove the flapjacks.
Put the butter, sugar, golden syrup and a pinch of salt in a pan, and gently melt over a low heat. Combine the oats, nuts, lemon zest and flour in a large bowl and pour over the melted sugary butter. Mix well, then fold in the blueberries. Tip the oat mixture into the lined tin and press to an even layer using the back of a metal spoon. Bake for 40-45 mins until golden brown. Leave in the tin to cool completely.
Lift the cooled flapjack onto a board. Put the white chocolate in a small bowl and melt in the microwave in 15-sec blasts, or over a pan of barely simmering water. Drizzle the melted chocolate back and forth over the flapjack. Leave to cool until set before cutting into squares.