Defrost frozen pie shell and mixed berries.
Preheat oven to 400°.
In a large saucepan, on medium heat, whisk together sugar, cornstarch, water, and butter until smooth.
Add 2 cups of mixed berries and stir gently for about 2 mins. on low heat. As soon as the berry filling becomes thick, turn off the heat, and let it cool.
Using a slotted spoon, gently scoop the berry filling into the pie shell. Leave any syrup sauce behind. The goal is not to make your pie filling watery. TIP: If you wish, you can store the berry syrup in a mason jar for up to 1 week. The syrup can used on pancakes or on top of ice-cream.
Place the pie onto a baking sheet pan. Bake for 25 mins. or until crust is golden brown. Remove from the oven and let it cool for about 15 mins.