Place the berries a colander over a large bowl and let thaw.
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.
Line a 9 inch pie plate with pastry crust. Refrigerate while prepping filling and topping.
Preheat oven to 375º.
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.
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.
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.
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.
Let cool an hour or more before serving.