Thaw the raspberries in a strainer placed over a bowl. About 1 cup of raspberry juice should drain out.
In a small bowl make a slurry with the water and cornstarch.
In a small saucepan gradually sweeten the juice to taste. Remember that the raspberries, when added back in, will make the sauce tarter.
Bring the raspberry juice to a simmer and whisk in about half of the cornstarch slurry. Keep whisking until the mixture returns to a simmer. Whisk in more slurry if needed to achieve the desired thickness.
Remove the saucepan from the heat, pour the thickened juice into a bowl and gently stir in the raspberries.
Let cool to room temperature. Refrigerate covered until cold.
If the sauce is too thick, stir in 1 teaspoon of water at a time until thin enough.
If the sauce is too thin, strain off some of the liquid into a saucepan and gradually thicken with a cornstarch slurry as above.