Combine the raspberries, sugar, and orange juice in a saucepan. Whisk the cornstarch into the cold water until smooth. Add the mixture to the saucepan and bring to a boil.

Simmer for about 5 minutes, stirring constantly, until the desired consistency is reached. The sauce will thicken further as it cools.

Puree the sauce in a blender or with a handheld immersion blender and strain it through a fine sieve. Serve warm or cold. The sauce will keep in the refrigerator for up to two weeks.