Whisk water and Clear Jel (or cornstarch) together in a small bowl to make a slurry and set it aside.
Place the blueberries and butter in a large pan over medium high heat and cook for about 1 minute. Pro tip - I like using frozen blueberries because they are always very ripe and tasty, and there’s no need to defrost them.
Add the sugar, water, and salt to the pan and stir them together.
Pour the Clear Jel (or cornstarch) mixture into the pan and cook on medium-high heat for 2-3 minutes until the mixture begins to bubble and thicken.
Cook for 1 more minute to make sure the Clear Jel is fully cooked. Remove the mixture from the heat and add the lemon juice and zest. Pro-tip – Cornstarch doesn’t like acid, so it’s best to add the lemon after your cornstarch has cooked out a bit.
Once it has thickened, remove the filling from the heat and pour it into a cake pan or heatproof bowl and cover with plastic wrap to avoid forming a skin. Store the blueberry filling in an airtight container for about 2 weeks in the refrigerator or freeze it for 6 months. Clear Jel will continue to thicken as it cools.
Use this recipe to make a 9" pie, make hand pies, or cut the recipe in half to use it as a cake filling.