wash & drain fresh raspberries (not necessary if they are frozen)
Place raspberries, sugar and lemon juice in pot and bring to a boil over medium heat
reduce heat and simmer for 3 to 5 minutes
use a hand masher or immersion blender to break up the berries completely
taste for sweetness and add more sugar if needed
Use a mesh sieve over a large bowl to strain out some or 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. This step is optional.
make a slurry of the corn starch and water
mix the slurry into the berries and continue to simmer for another minute stirring constantly
if it is not thick enough you can add another tablespoon of slurry and cook one more minute
remove from heat and pour into a heat resistant container
let cool uncovered. Store in refrigerator for up to 3 days or in freezer for up to 3 months.