In a medium sauce pan combine the frozen raspberries , water, lemon juice and sugar. Cook on medium heat and bring the mixture up to a boil.
Once all of the sugar is dissolved it is time to smash the berries. I find it easiest to blend it with an emersion blender. If you don't have one you can carefully transfer the mixture a blender or food processor .
Once the berries are blended return bring them to a medium heat and mix the corn starch and water to make a slurry.
Bring the sauce to a simmer and add the corn starch slurry . Cook until it is thickened
This is optional but if you don't like the seeds you will need to strain the mixture with a fine mesh strainer.
There will be quite a bit of seeds leftover Store raspberry sauce in the refrigerator in a sealed container