Mix together bajra flour, whole wheat flour, salt and one tablespoon oil in a bowl. Add sufficient water and knead into a soft dough. Cover and keep aside for ten to fifteen minutes.
To make stuffing, mix potatoes, onions, green chillies, red chilli powder, chaat masala, garam masala powder, salt and coriander leaves in another bowl.
Divide the dough into eight equal portions and the stuffing into four equal portions.
Roll out two portions of the dough into thick chapattis, dusting with a little flour. Spread a portion of the stuffing evenly on one of the chapatti and cover with the other chapatti.
Seal the edges and roll out into a thick parantha. Similarly make the remaining paranthas.
Heat a non-stick tawa. Place a parantha on it and cook for half a minute. Flip, drizzle a little oil all around it and flip again. Drizzle some oil all around again and shallow-fry the parantha till golden on both sides.