01 - In a medium saucepan, mix the berries, granulated sugar, and lemon juice.
02 - Heat the mixture over medium heat, stirring occasionally, until the berries release their juices and the mixture gently simmers, approximately 5 minutes.
03 - Whisk together the cornstarch and cold water in a small bowl until smooth.
04 - Slowly stir the cornstarch slurry into the simmering berry mixture.
05 - Continue cooking while stirring constantly until the sauce thickens and becomes glossy, about 2 to 3 minutes.
06 - Remove from heat. For a smooth sauce, blend using an immersion blender or pass through a fine sieve; otherwise, leave as is for a chunky texture. Allow to cool slightly before serving warm or chilled.