Mix cornstarch with 1 tbsp of water to form a slurry.
In a saucepan, combine berries, 1/2 cup of water, and agave, and bring to a boil. Add zest, lemon juice, and cornstarch mixture, stirring to combine. Continue to cook until thick. Turn off heat and allow to cool.
Chill overnight, or for at least 10 hours.