Wash and drain fresh raspberries (not necessary if they are frozen).
Place raspberries, sugar, lemon juice, syrup, half of the water and salt in a saucepan and bring to a boil over medium heat.
Reduce heat to medium-low and let the filling simmer for about 5 minutes.
Turn off heat and remove the pot from the stove. Pour the filling in a ninja or immersion blender and break the berries up completely.
Use a mesh sieve over a clean pot to strain out all of the raspberry seeds. Use the back of a large spoon to press all of the juice and pulp out of the seeds. Return to the sauce pan and bring back to a simmer.
In a separate bowl make a slurry of the corn starch and the remaining water. Stir until the cornstarch is fully dissolved into the water.
Mix the slurry into the berries and continue to simmer for another minute while constantly stirring.
When the mixture starts to boil remove from heat and pour into a heat resistant container and cover with plastic wrap.
Let it cool to room temperature. Then store in refrigerator for up to a week.