First of all make the potato bhaji. In a frying pan, put 1 tbsp oil and when hot put the mustard seeds till they crackle and also put the green chillies and curry patta. Then add the boiled and chopped potatoes along with salt and turmeric powder. Stir for few minutes and your bhaji is ready. Keep this aside and let it cool. Then make the besan batter by mixing the besan, soda, red chilli powder and salt. Don't make the batter too watery. It should be neither thick nor watery.
Now take one slice of bread. Then take some potato bhaji and spread it evenly on the bread. Place the second slice of bread on top of it so as to make a sandwich. Now cut this sandwich into four pieces. Then take each piece, dip it in the besan batter and fry till done. (Don't fry very brown). Keep this aside and let it cool. You can always fry this in advance and keep it because for the chaat you don't require hot. It has to be cool.
Lastly, place these fried bread bhajiya on a plate and cut each piece into another four pieces. Then put enough tamarind to soak into them. Sprinkle some cumin powder and red chilli powder. Then put the chopped onions and coriander leaves and lastly top it with sev.