Prepare the Chilies and Garlic: Wash and thoroughly dry the green chilies (hari mirch). Remove the stems and either pickle them whole or chop them into small pieces. Peel and thinly slice the garlic cloves. If you are substituting serrano or jalapeño peppers, slice them into thin rounds.

In a spice grinder or mortar and pestle, grind the mustard seeds with the pickle masala until they form a coarse powder.

In a mixing bowl, combine the chopped green chilies, minced garlic, ground spices, salt, and lemon juice. Mix well until all the ingredients are evenly combined.

Transfer the mixture to a clean, sterilized glass jar (see notes below about sterilization) with an airtight lid. Press the mixture down into the jar firmly with a clean, dry metal spoon. Close the jar tightly and store it at room temperature for two days. If you use thicker peppers like jalapeños, you may need to give it an extra day for the peppers to soften more.

After the fermentation period, add the oil. If you are using mustard oil, heat the oil to the smoking point and then allow it to cool for 20 minutes before adding it. This will reduce the bitter flavor of the oil. If you are using peanut or vegetable oil, heating the oil is not necessary. 

Open the jar and add mustard oil to the pickle mixture. Mix well to ensure all the ingredients are coated with the oil.

Once the mustard oil is mixed in, the pickle is ready to be enjoyed. However, it is recommended to let it rest for a day or two to allow the flavors to further develop. Store the pickle in the refrigerator and use it within two months. Remember to use a clean, dry spoon each time, so as not to introduce moisture or bacteria that could shorten the pickle’s shelf life.