Preheat oven to 450 degrees F.

In a bowl, combine strawberries, fruit, and 1/2 cup sugar. Mix cornstarch and lemon juice in a smaller bowl and drizzle over berry mixture, then stir to combine.

In a small mixing bowl, combine remaining 1/4 cup sugar and flour to make streusel topping. Using a pastry blender, two knives, or your fingers, cut or rub the butter into the sugar and flour mixture.

Pour berry mixture into pie crust. Sprinkle streusel over the top of the fruit. Place in 450 degree oven for 10 minutes, then reduce the heat to 350 degrees for 25 minutes, or until streusel is browned and the fruit is bubbling. Remove from oven and cool for at least 1/2 hour before serving.