In a medium-large bowl, combine the rhubarb, strawberries, sugar and cornstarch. Mix well. The mixture should be thick and syrupy after being stirred well.

Roll out pie crust according to recipe or package instructions and place in a 9- or 9.5-inch pie plate. Pour the strawberry/rhubarb mixture into the crust, using a large spoon to fill evenly.

For the topping: In a small bowl, combine the flour and brown sugar. Cut the butter into pieces and using a pastry cutter or two knives, cut the butter into the flour/sugar mixture until it has the consistency of coarse crumbs. Sprinkle the streusel topping evenly over the top of the pie.

Place the pie on a rimmed baking sheet and bake at 400 degrees for 50-55 minutes, covering the pie crust edges with foil halfway through in order to prevent over-browning.

Let the pie cool completely before cutting into pieces. Serve with a dollop of freshly whipped cream, if desired.