Add cherries to the bowl of a food processor. Sprinkle with ¼ cup sugar (or monkfruit sweetener) and lemon juice.
Process until smooth and sugar has dissolved, then taste the mixture to see if it needs additional sweetener (this will depend on the sweetness of your cherries and your personal preference). If so, add an additional tablespoon or so of sweetener, then pulse to incorporate.
Pour mixture into a Ninja Creami pint container and close its lid. Transfer pint to a level surface in the freezer for at least 24 hours.
Remove container from freezer and take off its lid. Place pint into the outer bowl of the Ninja Creami, securing the outer bowl lid. Transfer bowl apparatus to the Ninja Creami machine, locking it into place. Press the Sorbet button and allow the cycle to complete.
If the mixture appears crumbly after the first spin, you can insert it back into the machine and use the Re-spin button.
Serve and enjoy!
