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Preheat oven to 350 degrees. Using softened butter, grease a 13x9" baking dish; set aside.
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In large bowl, toss blueberries with sugar, cornstarch, 2 tablespoons flour, lemon juice, water, and vanilla until blueberries are well coated.
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Spread blueberries into prepared baking dish.
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In medium bowl, blend 1 1/2 cups flour, salt, oats, brown sugar, and cinnamon.
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Melt butter in microwave, then pour melted butter into oat mixture. Mix well.
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Use fingers to crumble oat mixture over top of blueberries. Bake at 350 degrees for 35-40 minutes until mixture is bubbly and topping is golden brown.
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Let cool for at least 30 minutes; serve warm with vanilla ice cream.