01 - In a medium saucepan, combine the mixed berries, sugar, and lemon juice.
02 - Place the saucepan over medium heat and stir occasionally until the berries release their juices and the mixture gently simmers, approximately 5 minutes.
03 - In a small bowl, whisk the cornstarch and cold water until smooth.
04 - Stir the cornstarch slurry into the simmering berry mixture.
05 - Continue cooking over medium heat, stirring constantly, until the sauce thickens and develops a glossy appearance, about 2 to 3 minutes.
06 - Remove from heat. For a smooth texture, blend with an immersion blender or strain through a fine sieve to remove seeds; otherwise, leave chunky.
07 - Allow to cool slightly before serving. Serve warm or chilled according to preference.