After about 5 minutes, add the smashed potatoes also and mix it well.
Masala for Masala dosa is ready. Keep it aside.
Soak about 4 cups of raw rice (pachari or puzhukkalari) along with 1 cup of black gram dal (uzhunu parippu) and 3 – 4 spoons of fenugreek seeds for about 5 – 6 hours.
After 5 – 6 hours, wash the soaked mix of rice, black-gram dal and fenugreek seeds and grind them together in a mixer or wet grinder for about 15 minutes and make the batter (as shown in the below picture).
Add 2 – 3 tsp of salt with the batter, mix it well and allow to ferment the batter for about 6 – 7 hours time. (Before making dosas, the batter should stir for about 1 – 2 minutes).
Heat the dosa tawa and pour one ladle or about half a cup of the already prepared batter (depending on the size and thickness of dosa) on the tawa and spread the batter on round shape as shown in below picture.