Soak the parboiled rice and raw rice for 3 hours together.
Soak the dals and fenugreek for 3 hours.
Drain and grind them separately to a fine batter.
Add salt and mix both the batters.
Keep aside for 8 hours or overnight to ferment.
To make the potato masala, heat oil in a pan over medium flame.
Fry the mustard seed, urad dal and bengal gram dal for 30 seconds.
Add the curry leaves, onions, ginger and green chillies.
Add turmeric powder and stir well.
When the onions turn light brown, add the potatoes and salt.
After 2 minutes, add the coriander leaves and mix. Remove and keep aside.
Heat a tawa over medium flame.
Pour a ladleful of the batter and apply oil around the edges.
Cook on one side and flip it over.
Add 2 tblsp of the potato masala and fold the dosa lightly.
Do not fold fully.
Cook for a minute or two and transfer to a serving plate.
Serve hot with chutney and sambar.