In a medium sauce pan bring water to a simmer. Add dried hibiscus and bring back to a boil, stirring to soak all the hibiscus. Turn off heat and allow to steep for 10-15 minutes, stirring once or twice. Strain through a fine mesh sieve, allowing hibiscus to drain naturally without squeezing or pushing on it.
Return liquid to the pot and add frozen strawberries along with any juice and sugar. Bring to a boil and stir to dissolve sugar. Reduce heat to a steady boil and allow to cook, uncovered for 15-20 minutes, mashing berries with a fork or back of a wooden spoon to help them break down.
Mix cornstarch with 1 tbsp cold water until dissolved. Add to syrup, stirring constantly until mixture is fully combined and has returned to a boil. Strain through a fine mesh sieve and allow to cool to room temperature, then cool completely in the refrigerator to reach full thickness.
Pour into sealable jars or airtight container and keep refrigerated until ready to use. Will keep refrigerated for 2-3 weeks well sealed.