In a medium skillet, stir the granulated sugar and water over moderately high heat until the sugar dissolves. Add the mixed berries and lemon juice and cook, gently stirring occasionally, until the berries are heated through and begin to release their juices, about 2 minutes. Drain the berries and return the juices to the skillet. Cook the juices until reduced by half. Let the syrup cool slightly, then gently fold it into the berries. Refrigerate until chilled, about 20 minutes.