Optional: 1 cup fresh or frozen bing cherries(pitted - pits removed)
In a small saucepan, combine the cherry juice and the sugar, and bring to a boil, stirring a few times to help the sugar melt and prevent burning. If you want to add some cherries to this syrup, add fresh cherries now, or frozen cherries in step 3.
Lower the heat and let simmer uncovered for 15-20 minutes, or until the syrup coats a spoon or spatula and thickens to your liking (it will continue to thicken after it cools!).
Optional: stir in fresh lemon juice and/or frozen bing cherries.
Carefully taste (it's hot!! allow it to cool on a spoon first!) and adjust the sweetness by adding 1-2 tablespoons more sugar, if needed. Allow to cool to room temperature, then transfer to a jar and store in the freezer for up to 3 weeks.
If using sweetened cherry juice instead of tart juice, reduce the sugar. If you reduce the sugar, the syrup will not thicken as much and you might want to thicken the syrup with a cornstarch slurry (mix ½ tablespoon of cornstarch with cold water until no clumps remain and whisk into the syrup during the last few minutes of cooking).
Try adding a sprig of rosemary or two sprigs of thyme to the syrup as it's cooking for a sweet & savory cherry syrup (wash the herbs thoroughly first!). Or add a slice or two of peeled ginger for a slightly spicy gingery syrup.