Dice the potatoes into 1/2 in cubes. Heat the oil in a skillet over a medium heat . Add the cumin, mustard and green chillies. When the mustard starts to splutter, add the potatoes, turmeric, chilli, coriander and salt. Stir fry for 5-7 minutes. Put aside. Cool.
Take 2 cups whole wheat flour or corn flour, 1tbsp oil/ghee, 1 tsp salt, warm water and oil for deep-frying. Mix flour, salt, oil/ghee and make a tight dough.
Keep aside for 20 minutes.
Divide the dough into balls (nearly equal to tennis ball size).
Take a ball, flatten it on a slightly floured surface and roll into a round of 3" across. Place 2 tbsp of the filling in the center and fold up the edges enclosing the filling completely. Press the ball (bati) lightly in between your palm (to make bati little flate).
Take water in a saucepan for boiling. Put the bati into boiling water and boil it on medium heat for about 30 minutes.
Take out bati when done and keep aside, till it cools (about 30 minutes).
Heat the oil in a saucepan over a medium heat. Put as many batis as you can into the hot oil. Fry until crisp and golden. Drain.