Wash and dry 500 grams of fresh cherries. Remove the stems and pits, and cut them into halves.
In a pan, dry roast 1 teaspoon of mustard seeds, 1 teaspoon of fenugreek seeds, and 1 teaspoon of fennel seeds until fragrant. Allow them to cool and then grind them into a coarse powder.
In a separate pan, heat 100 ml of mustard oil until it reaches its smoking point. Turn off the heat and let it cool down to room temperature.
In a large mixing bowl, combine the cherries, ground spice mixture, 1 teaspoon of turmeric powder, 1 teaspoon of red chili powder, 1 teaspoon of salt, and the cooled mustard oil. Mix well to ensure the cherries are evenly coated with the spices and oil.
Transfer the cherry mixture to a clean, sterilized glass jar. Seal the jar tightly and let it sit in a cool, dark place for 3-4 days, allowing the flavors to develop.
After 3-4 days, the cherry achar is ready to be served. Store it in the refrigerator to extend its shelf life.