Soak the seedless tamarind in 1/4 cup boiling water. Allow this to soak for 10-15 minutes. Once it has softened, mash the tamarind using your fingertips and make a thick paste. Remove seeds, if any.

Clean the green chilies and wipe them dry using a cotton kitchen cloth. Give them a rough chop.

Now take a large blender jar and add the tamarind, chopped ginger, green chilies, and half of the chopped yam or ol (suran/jimikand). Place the lid on the jar and fasten it. Blend it on low speed by giving it regular stirs at intervals. Scrape the sides and push them towards the center at the blades and give it a blend.

Add the turmeric powder, salt, garlic, and the rest of the chopped yam pieces. Give it a blend again until you get a fibrous mix. Adding garlic is optional. You can omit it in case you do not prefer it.

Take it out in a large mixing bowl and give it a mix. If you find larger pieces of yams or ol, run them again in the blender. Once everything has turned into a fibrous yet homogenous mix, pour the mustard oil so that you get an oily mix.

Spread the mix uniformly in a glass mixing bowl or a dish and place it somewhere it would get direct sunlight for most of the day. Keep giving it a mix every hour or so, as it will form a brownish crust due to drying.

At the end of the day, give it a last mix and pour some mustard oil, forming a thin covering layer. Cover the bowl and place it in a cool and dry place.

Repeat this process of sun drying and the addition of oil for the next 7-8 days until the achar has dried up considerably and darkened in color.

When it's ready, transfer to a clean, dry glass jar. Continue keeping the jar in sunlight for the next couple of days (without the lid) to allow the flavors to mature further.