In a separate cup, whisk water and ClearGel (or cornstarch) together to make a slurry, set aside.
Place the blueberries in a large pan over medium heat. Add butter and cook about 1 minute.
Add sugar, water, salt and ClearGel slurry. Cook and stir for 2 to 3 minutes on medium-high until the mixture begins to bubble.
When it starts bubbling, cook for 1 more minute on medium heat to make sure the cornstarch is fully cooked. Add the lemon juice and lemon zest and stir to combine.
Remove the mixture from the heat and pour it into a cake pan or heat-proof bowl and refrigerate to cool quickly before using. Store about 2 weeks in the fridge or freeze for 6 months. The filling will continue to thicken as it cools.
If you want extra shiny, beautiful filling, use ClearGel instead of cornstarch. This is what bakeries use in their fruit fillings.
Cornstarch is much stronger than ClearGel so if you're using cornstarch, cut the amount in half (for every 1 ounce of ClearGel, use 1/2 ounce of cornstarch)
Make sure to cook your filling for 1 minute after it begins to bubble to ensure it's fully thickened and the ClearGel is cooked
Cornstarch doesn't like acid, so it's best to add the lemon after your cornstarch has cooked out a bit.
Serving: 4ounces | Calories: 371kcal (19%) | Carbohydrates: 81g (27%) | Protein: 1g (2%) | Fat: 6g (9%) | Saturated Fat: 4g (20%) | Cholesterol: 15mg (5%) | Sodium: 212mg (9%) | Potassium: 366mg (10%) | Fiber: 6g (24%) | Sugar: 64g (71%) | Vitamin A: 300IU (6%) | Vitamin C: 13mg (16%) | Calcium: 50mg (5%) | Iron: 1mg (6%)