Cut the potatoes in half, leaving on the skin. Boil them until cooked and then drain and leave to cool. When cool enough, use your hands to roughly 'mash' the potatoes, leaving with your some in chunks and some more mashed.
Heat oil or ghee large frying pan or saucepan and add mustard seeds.
When the mustard starts popping add chopped onion, curry leaves and turmeric. Saute for around 10 minutes.
Add salt and 125ml cup water and stir well.
Add the mashed potato, coriander leaves, ghee and mix well. Turn off and set aside.
To make the dosas, put a non-stick frying pan on a medium to high heat.
Put a few drops of ghee on the fryingpan, using some cloth to rub it all over the pan. You need to repeat this for each dosa you make.
Use your fingers to sprinkle a little bit of water on the hot frying pan and wait for the water to crackle and evaporate (this cools the pan slightly so the dosa will spread evenly and not break). You need to repeat this for each dosa you make.
Gently pour 1/4 cup (~60ml) of fermented dosa batter into the center of the frying pan.
Use sweeping circular movements to spread the batter into a circle (I use a silicon spatula). Small holes will appear in the pancake, this is all part of the process.
Apply a little ghee over the surface of the dosa and around its edges.
When the edges start to turn dark brown (after 1-2 minutes), add some masala potatoes to the centre of the dosa and then flip two sides of the dosa over each other.
Eat before someone else grabs it!