In a small sauce pan, combine 2 cups of the blueberries, sugar, lemon juice, all but 1 tablespoon of water.
Bring to a boil for 10 minutes until blueberries burst.
Combine cornstarch and water, then whisk it into the berry mixture. Bring to a boil, whisking constantly for 4-5 minutes until thick.
Add the remaining 1 cup of blueberries, cover and set aside.
Notes
Feel free to add more or less sugar, depending on your personal preference. Sugar alternatives work well, too.Stir the cornstarch with cold water before adding it to the blueberries. This prevents clumping and ensures the sauce thickens smoothly.Keep the heat on medium and stir frequently to prevent the sauce from burning or sticking to the pan. This will also help the sauce thicken evenly.