Blueberry Galette

Ingredients

• 1/4 cup sugar
• 1/2 cup butter (cold, cut into cubes)
• 2 tablespoons cornstarch
• 4 cups blueberries
• 1 tablespoon lemon juice
• 3 tablespoons water (ice cold, divided)
• 1 large egg
• 1 tablespoon sugar
• 1 1/4 cups all-purpose flour