In a small saucepan, whisk together sugar, 3 tablespoons (45 grams) water, cornstarch, and salt until smooth. Stir in 1½ cups (222 grams) blueberries, and bring to a boil over medium heat. (Berries should burst, and mixture will become dark purple-blue.) Cook, stirring constantly, for 2 minutes. Remove from heat, and stir in butter, lemon juice, and remaining 1½ cups (222 grams) blueberries. Let cool completely before using.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/may-blueberry-cornmeal-galette/