Heat the oil on medium heat and add the black and yellow mustard seeds. Once they making a popping sound add nigella seeds, fennel seeds, dried red chilies, curry leaves and garlic. Saute until the garlic starts to turn lighly golden.
Add the carrots, along with the ground spices and salt. Mix well and cook for 10 minutes, stirring frequently.
Add the vinegar and quickly mix before turning off heat.
Let the achaar cool down to room temperature before transferring it to a clean and dry glass jar. Keep it stored in a refrigerator and consume within 2 weeks.