In a large bowl, combine the berries, rhubarb, sugar and cornstarch. Mix well. The mixture will start to turn thick and syrupy as it is stirred.
Roll out the pie crust and place in a 9-inch pie plate, trimming and fluting the edges.
Pour the berry/rhubarb mixture evenly in the crust.In a small bowl, combine the flour and brown sugar.
Add the butter, and using a pastry cutter or two knives, cut in the butter to the flour/sugar mixture until it has the consistency of coarse crumbs.
Sprinkle the streusel topping evenly over the top of the pie (but not covering the edges of the pie crust).
Place the pie on a foil-lined baking sheet (to catch any drips), and bake for 70-75 minutes until the streusel is golden and the filling is bubbling thick and syrupy. Cover the pie crust edges halfway with foil or a pie crust shield to prevent over-browning, if needed.
Let the pie cool completely before cutting (the filling will thicken as it cools). Serve with a scoop of vanilla ice cream or sweetened, whipped cream, if desired.
Notes
Pie Crust: My favorite homemade pie crust is this one or this one.Triple Berry Blend: I prefer using a triple berry blend of blueberries, raspberries and blackberries.