Boil potatoes either in a pot or pressure cooker.

If cooking in a pot, bring water to a boil and add the diced potatoes. Cook them just until done or fork tender and not mushy. OR If cooking in a pressure cooker, add the potatoes to a bowl and pour little water. Cover and pressure cook for 3 whistles on a medium flame.
Ensure potatoes are just cooked and not very soft or mushy.

Heat oil in a pan and add mustard and cumin.When they begin to splutter, add chana dal and urad dal.

Fry until golden and then add hing.
Add onions, green chili and curry leaves.
Saute till the onions turn transparent. Then add ginger and stir for 30 seconds.

Once the onions turn transparent, add potatoes, turmeric and salt..
Mash slightly and add very little water or potato boiled stock to make the entire potato masala moist.

Stir and cook for one to 2 mins.
Add coriander leaves. Stir. Serve potato masala with dosa.

Now heat an iron tawa or a pan. dip a paper napkin or half of an onion in oil and spread the oil all around on the tawa.

Please use a heavy iron pan as this dosa takes more time to cook.
keep the tawa on a low flame. then take a ladle full of the dosa batter.
spread dosa batter to a neat round circle.

On a low flame cook the dosa till the top looks cooked.
as soon as the top portion of dosa looks cooked, add 1 tablespoon of Aloo sabzi.

Cover with a lid and cook for 30 to 45 seconds. do keep the flame to a low or sim.

Begin to cut the dosa with a knife, steel spatula (palta) or pizza cutter. since the dosa is cut directly on tawa, an iron tawa works best.

If you do not have an iron tawa, then just roll the whole dosa. take it in a steel plate or a chopping tray and then cut the dosa with knife.

You can cut the dosa in three to four parts.