If you don't want to start with the cordial, heat the frozen berries without water until warm, mash with a fork or blend 3/4 of the berries until somewhat smooth. I like to leave some bigger chunks if possible.
36 oz worth of mashed warmed raspberries
1 - 2 cups sugar (to taste)
Juice and zest of 1 lemon
1/4 tsp salt
Add all ingredients to stock pot and bring to a simmer.
Simmer for 15-20 minutes until reduced by a third and it has become thicker and syrupy, to your desired consistency. I like mine pretty thick, but pourable when warmed for 10 seconds in the microwave.
Pour into clean sanitized jars. You can see this made quite a bit, aprroximately 24 oz.