Preheat oven to 375°F. Place baking sheet in the oven. Combine mixed berries in a bowl. In a separate bowl, mix together the the sugar, corn starch and salt. Then combine with the berry mixture and stir together.
Mix the crumble topping ingredients together in a bowl. You may need to use your hands to combine. Then set aside.
Make crust according to pie crust directions. Add the berry mixture on top (leaving any excess juices in the bowl). Sprinkle the streusel topping over the top of the berries.
Place pie on top of baking sheet in oven. Bake pie for 40-50 minutes, or until top is golden brown and the center is firm. Add foil to cover the crust if it is browning too fast. Remove from oven and allow to cool (3-4 hours) to thicken before slicing.
Serve with fresh whip cream or vanilla ice cream.
Berries: You can use fresh or frozen berries for this pie. I prefer to use more raspberries (about 2 cups raspberries, 1 cup blackberries and 1 cup blueberries). For frozen mixed berries, take out the strawberries and allow to thaw slightly. Drain any excess juices.Pie crust: You can buy a pre-made pie crust (I prefer the one in the freezer section) or make half a batch of my favorite pie dough: https://www.iheartnaptime.net/easy-pie-crust-recipe/