In a large saucepan, whisk sugar, cornstarch, salt, water and some cinnamon (optional) until a smooth mixture forms.
Add in some fresh blueberries and let the mixture boil, then cook and stir it for 2 minutes until it reduces and thickens at a medium flame. Let it cool down for a while before you get to the next step.
Preheat your oven to about 400℉ and while that heats up, add, start gently folding strawberries, blackberries, and lemon juice into the blueberry mixture.
Then, lightly flour a surface, roll out the larger portion of the dough to a 1/8-inch-thick circle and transfer it to a 9-inch pie plate and trim the edges of the crust ½ inches beyond the rim of the plate.
Next, add the filling and dot with butter.