Add 12 ounces frozen raspberries to a medium saucepan and heat over medium-high until berries break down, about 10 minutes.
Bring raspberries to a light boil, stir in 1 cup granulated sugar, and return to a boil.
In a separate small bowl, whisk together 3 tablespoons corn starch and 2 tablespoons water and pour into the raspberries.
Stir the mixture vigorously for 1 minute and then remove from heat and let cool and thicken, stirring occasionally.
Bottle up and store in the fridge or freezer, or use right away.
Notes
To Freeze: let it cool completely, then transfer it to a freezer-safe container and fill it 3/4 of the way full. The best way I have found is to lay freezer bags flat in the freezer, so they're nice even packs.
The filling should keep in the freezer for 3 months or in the fridge for about 1 week.
If using fresh raspberries, you may need to add an additional tablespoon of cornstarch to thicken it.