Pit cherries and roughly chop by hand or in a food processor.
1 pound fresh cherries
In a saucepan, combine cherries, onion (or shallot), brown sugar, vinegar, salt, cayenne, and cloves, stir, and heat to a boil over medium-high.
1/4 cup diced onion, 1/4 cup brown sugar, 2 tablespoons red wine vinegar, 1 teaspoon fine sea salt, 1/4 to 1/2 teaspoon ground cayenne pepper, 1/8 teaspoon ground cloves
Reduce heat to medium-low and simmer, stirring frequently, until cherries and onions are soft, about 25 minutes. Remove from heat and cool briefly.
Transfer sauce to a blender or food processor and blend until completely smooth, making sure the lid is vented to allow steam to escape.
Pour into a jar or other airtight container. Let cool to room temperature, then store in the refrigerator for up to 10 days.
Notes
Makes about 2 cups.
Sauce will thicken more upon standing, but you can also simmer the blended sauce until it has thickened to your liking.