Wash and dry 500 grams of brinjals. Cut them into small pieces and keep aside.
In a pan, dry roast 2 tablespoons of mustard seeds, 1 tablespoon of fenugreek seeds, and 1 tablespoon of fennel seeds until fragrant. Let them cool and then grind them into a coarse powder.
In a separate pan, heat 250 ml of mustard oil until it reaches its smoking point. Let it cool down to room temperature.
In a large mixing bowl, add the brinjal pieces, ground spice powder, 2 tablespoons of red chili powder, 1 tablespoon of turmeric powder, 1 tablespoon of asafoetida, and salt to taste. Mix well.
Pour the cooled mustard oil over the brinjal mixture and mix thoroughly, ensuring all the pieces are well coated with the oil and spices.
Transfer the mixture to a clean, dry, and 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 Baingan Achar is ready to be served. Store it in the refrigerator for longer shelf life.