Heat peanut oil. Switch off the flame. Let the oil come to a warm temperature. Keep it aside.
Take split mustard seeds in a blender jar and grind them in a blender to make a coarse powder.
Add crushed mustard seeds, split fenugreek seeds, fennel seeds, and warm oil to a big bowl. Mix everything very well.
Now add lemon juice and mix everything again. Keep this aside for about 4 to 5 hours.
Wash and completely dry the chili peppers. Please make sure no traces of water exists on them. Cut the top of the chilies and cut them lengthwise.
Take cut chillies in a big bowl. Add turmeric powder and salt. Mix well.
Cover the bowl and keep it aside for an hour.
This step will release the excess water present in chilies and also reduce some spice from the chilies.
Once one hour is done, take out the chilies from the bowl with the help of a perforated ladle and spread it in a single layer on a clean kitchen napkin or paper towel. Discard the water (which is released from chilies).
Let the chillies air dry on a countertop for about 5 to 6 hours.
Once the chilies are dried, take them into a big bowl. Add in the Mustard-fenugreek mixture and mix everything very well.
Cover the bowl and keep it on the countertop for 3 to 4 days. Stir once every day.
Raita marcha is ready to enjoy. Take them in a clean and dried pickle jar/bottle and store them on the countertop or in the fridge.
Serve it with Thepla, dal-bhaat, paratha, roti, or with any meal.