In a saucepan over medium heat, combine the fresh blueberries, sugar, and lemon juice.

Cook for about 5-7 minutes, stirring occasionally, until the blueberries start to release their juices and the mixture begins to bubble.

If using cornstarch, mix it with water in a small bowl to create a slurry, then add it to the saucepan.

Continue to cook for another 2-3 minutes until the sauce thickens slightly.

Remove from heat and let cool slightly before serving over cheesecake.