Add cherries, sugar, and a splash of water to a saucepan and heat over medium heat.
Stir until the sugar dissolves, and let the cherries get soft and release liquid. If anything starts to stick to the bottom, add more water. Cook for about 10-15 minutes.
Mix 1 tbsp of cornstarch with 1 tbsp of water, and add this to the sauce. Make sure to heat the mixture up to a boil so that the cornstarch works to thicken the sauce.
If the mixture is still very runny and not thick enough, repeat the previous step with 1 more tbsp of cornstarch.
If you find the mixture is too sweet (especially if using sweet cherries), we've found that adding lemon juice, pomegranate juice, sour cherry juice, or another tart juice can help balance the sweetness!
Once the sauce is nice and thick, set it aside and then in the fridge to cool. Reheat to serve.