Heat a saucepan over low/medium heat.

Stir 2 cups of blueberries, sugar, lemon juice, 3 tablespoons water into pan and bring to a boil.

Let boil for 3-4 minutes, then add remaining 1 cup of blueberries. Stir.

Mix 4 tablespoons water and corn starch together. Add to blueberries and stir.

Continue cooking until sauce thickens.

Remove from heat and either serve warm over pancakes, etc., or cool and use for toppings on cheesecakes, etc.