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1
Preheat the oven to 180C/350F and grease and line an 8x8 inch square pan.
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2
Place the butter, flours, icing sugar, lemon zest and salt into a large bowl or the bowl of your stand mixer.
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3
Mix on med-high for 2-3 minutes until you have a crumbly mix, with no large lumps of butter. This dough will not come together like a normal dough, that's fine!
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4
Tip the shortbread into your greased pan. Spread and level with a spatula/the back of a wooden spoon.
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5
In a separate, medium sized bowl, place the 1/2 stick of butter, flour, sugar and oats.
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6
Mix together with your fingers, until the mix resemble bread crumbs. Some larger lumps are fine. Leave to one side.
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7
Place the blueberry jam into a small heatproof bowl and heat for 10-15 seconds. This just makes it easier to spread.
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8
Pour over the shortbread and spread until level. Leave a small 1/2 inch gap around the edges.
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9
Sprinkle the jam layer with the crumble mix, it's fine if there are a few gaps.
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10
Place in the oven for 35-40 mins until the top is lightly browned, the jam is bubbling and an inserted skewer into the middle comes out piping hot.
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11
Leave to cool completely in the pan.
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12
Once cooled, slice into 9-12 bars. Bars will keep in an airtight container, at room temperature for 3 days.
