Mix Besan, buttermilk, salt, turmeric powder, ginger and green chili paste and lemon in a bowl.Take cake not to form any lumps.If a few lumps are there, just strain the mixture through a sieve.Grease a smooth work surface very lightly. (I used my kitchen counter. You can also grease the back side of steel plates or a baking tray. )Heat the mixture on medium heat till it thickens to the right consistency. ( It will take approx 8-10 minutes. To test if the mixture is done, pour a drop of it on a plate and spread it a little bit and let it cool. Now try to roll it. If it rolls, the mixture has reached the right consistency. Otherwise keep cooking it a little more. )Keep mixing continuously otherwise the mixture will burn from the bottom.Pour the mixture on the greased platform.Quickly spread the mixture in a thin layer using a spatula or spoon.Let it cool for a few minutes.Cut into thin stripes. ( If you are a maniac like me, use a ruler to get equal size stripes :) )Roll the stripes.For tadkaHeat oil in a pan.When the oil is hot, add mustard seeds.When the seeds starts crackling, add sesame seeds, heeng, curry leaves and green chili.Pour the tadka over the khandvis.Garnish with fresh coriander and grated coconut.