Combine the water, cornstarch, and lemon juice and set aside.
Stir together the blueberries, granulated sugar, and brown sugar. Bring the mixture to a boil over medium heat, stirring often.
Reduce the heat. Add the ground cinnamon and the cornstarch and water mixture. Simmer and stir until the sauce is thickened, about 2-3 minutes.
Remove from heat and allow the filling to cool.
Video
Notes
One: Fridge
Store the cooled filling in airtight containers in the fridge for 2-3 days.
Two: Freezer
Allow the filling to cool, and then transfer it to freezer-safe containers or zip-top freezer bags. Be sure to leave a little headspace in the containers so that there is room for expansion. If using zip-top freezer bags, remove as much as possible. Store in the freezer for up to six months.