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Mix the butter, flours, rolled oats, sugar, salt and baking powder in a large bowl until it forms crumbs.
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Set 1 cup of the mixture aside for the topping.
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Mix the egg white into the mixture.
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Press the mixture into the bottom of a greased 8 inch square baking pan.
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Bake in a preheated 350F oven for 10 minutes.
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Meanwhile mix the egg yolk into the sweetened condensed milk, lemon juice and zest and let it sit.
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Spread the blueberries out over the bottom crust and top with the lemon mixture.
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Bake until the lemon mixture forms a shiny skin, about 7-8 minutes.
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Sprinkle on the reserved crumbs.
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Bake until the top is golden brown and the sides are bubbling, about 20-25 minutes.