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Place the berries a colander over a large bowl and let thaw.
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Make crumb topping by mixing together flour, baking powder, brown sugar and cinnamon in a medium bowl. Mix in melted butter till combined. Set aside.
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Line a 9 inch pie plate with pastry crust. Refrigerate while prepping filling and topping.
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Preheat oven to 375º.
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Take one cup of berry mixture and add them along with the sugar to a saucepan. Cook over low heat till they start to boil.
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Mix cornstarch with a couple tablespoons of water and add to the fruit along with about a half cup of berry juices from the bowl beneath the colander. Bring to a boil and cook till mixture thickens and becomes clear.
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Pour berries from the colander into bowl. Mix in thickened berries and juices, then pour the berry mixture into pie crust. Crumble topping over filling.
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Bake at 375º for 15 minutes. Decrease temperature to 350º and bake another 45 minutes till filling is bubbling. Cover with foil if topping starts to get too dark.
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Let cool an hour or more before serving.