Wash blueberries, check for rotten ones and remove the stems if any.
Place blueberries in into a medium saucepan.
Add sugar, lemon juice, and lemon zest, then cook over medium-low heat for 5-10 minutes, stirring occasionally until the blueberries soften and release their juices.
Mash the blueberries with a spoon or potato masher to your desired texture.
In a small bowl, mix cornstarch with water to create a slurry, then add it to the blueberry mixture, stirring constantly.
Continue to cook the mixture until it thickens, which should take 2-4 minutes.
Remove from heat and allow the filling to cool for 15-20 minutes before using.
Notes
Storage:Store in an airtight container and keep in the refrigerator for up to one week or freeze for 3 months.