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In a bowl, combine blueberries, sugar, cornstarch, lemon juice and lemon zest. Transfer to a baking dish.
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In a mixer, cream butter and sugar until pale and fluffy. Stir together flour, oats, baking powder, and salt and combine with butter mixture. Squeeze topping pieces together into clumps and sprinkle over blueberry filling.
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Bake for about 1 hour or until brown on top and bubbling in center.