This sauce is quite versatile. It can be made ahead of time, covered, and refrigerated. You can serve it cold or at room temperature.
Drain raspberries, reserving syrup. Press raspberries through a sieve into a bowl, reserving purée; discard seeds. Combine water and cornstarch in a saucepan; stir until blended. Stir in reserved raspberry syrup and purée; bring to a boil, and cook 1 minute, stirring constantly. Pour into a bowl; cool.