There's a pie company in Michigan called Achatz, and they make the best Michigan Four-Berry pie. This recipe rivals the pie at Achatz and is pretty easy to make. In my book, The Essence of Shade, the main character makes a Triple-Berry pie for her grandson, Tyler--one of his favorite desserts. You can make it all year by using frozen berries. Costco sells a large bag at a pretty decent price. The outcome is well worth the effort!
Preheat oven to 400 degrees. Roll out pie crust and place in bottom of 9-inch pie plate.
Add frozen berries, sugar, and lemon juice to large saucepan and stir gently over medium heat for about 5-10 minutes until juices are released. Remove the pan from the heat and sprinkle in the cornstarch, a little at a time, stirring constantly and vigorously to avoid lumps. Return to the heat and simmer for about 5 minutes, stirring gently until thickened. Remove from the heat and stir in the butter. Allow to cool slightly while making the crumble topping.
For the crumble topping, combine all ingredients except butter in a large mixing bowl and mix well. Add butter pieces and work in with your fingers until mixture resembles coarse crumbs.
Pour fruit filling into unbaked pie shell. Sprinkle crumble topping over the top of the fruit filling, covering entirely.
Place prepared pie on foil lined baking sheet to capture spillage. Cover pie edges with foil to prevent burning. Bake for 45-55 minutes. If crumble topping begins to brown to quickly, cover loosely with foil.
Cool completely before serving and store leftover in the refrigerator. Let it come to room temperature before serving. Serve with vanilla ice cream, if desired!