Add the red raspberries and sugar to a large saucepan and cook over medium-high heat, stirring occasionally, until raspberries begin to release their juices.

In a small bowl, whisk together the cornstarch, water and lemon juice. Add the mixture to the cooking raspberries. Bring sauce to a boil and cook for one minute more. Remove from heat and allow to cool to room temperature before storing in an air-tight container in the refrigerator.