Combine the fresh pitted cherries with the sugar in a large bowl and let the mixture stand for about 5 hours at room temperature, stirring occasionally, until the sugar is dissolved and the cherry juices are released
Drain well in a wire strainer set over a bowl.
Transfer the juice to a 2-cup glass measure and add enough water (if necessary) to reach 1 1⁄4 cups.
In a heavy-bottomed 2-quart saucepan, whisk together the liquid, cornstarch, and salt.
Bring to a boil over medium heat, stirring gently but continuously with a rubber spatula, and cook 2 to 3 minutes, or until the mixture is thick and translucent. As the liquid heats, you’ll see that it forms small red clots.
The temptation is to stir or whisk rapidly to make the mixture smooth. Don’t! Just keep stirring gently as more and more clots form and before you know it, they’ll all join together to make a sauce that is thick and smooth. Rapid and vigorous stirring of cornstarch sauces may make them thin out instead of thickening, so be gentle.
Stir in the lemon zest and drained cherries.