Place the raspberries in a medium pot over medium heat and cook for 5 minutes or until raspberries are thawed and starting to break down.
Add the sugar and jam and bring to a simmer. Cook for 3 minutes.
In a small bowl, whisk together the cornstarch and water until smooth. Add the cornstarch to the raspberry mixture and cook for 1 minute or until sauce has thickened. Cool, then enjoy!
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Notes
If you don't want any seeds, simply let the sauce cool a bit and strain the sauce through a sieve to remove seeds.
This recipe makes enough sauce to serve a crowd, but can easily be halved if you're serving a smaller group.