Place the diced potatoes in a pot, cover with water, and boil until fork-tender.
Drain the potatoes, making sure to reserve about 1 cup / 240ml of the starchy cooking water. Gently crush the boiled potatoes with a fork or masher for a coarse, chunky texture.
Heat the oil in a medium nonstick frying pan over medium heat.
Once hot, add the chana dal and toast for 15 seconds, then add the urad dal. As soon as the dals turn a light golden brown, add the mustard seeds.
When the seeds begin to pop, quickly add the crushed curry leaves—be careful, they will splutter!
A few seconds later, add the diced onion, ginger, and green chili.
Sauté for 3-4 minutes until the onions start to soften.
Pour in 5 tablespoons / 75ml (2.5 fl oz) of the reserved potato water, cover the pan, reduce the heat to low, and cook for 3 minutes until the onions are fully soft and translucent.
Add the coarsely crushed potatoes, salt, and turmeric to the pan. Gently stir everything together over low heat for 2-3 minutes, allowing the potatoes to absorb the flavors.
Pour in another 6 tablespoons / 90ml (3 fl oz) of the reserved potato water and continue to stir for 2 more minutes. The mixture should be moist but not watery.
Finally, turn off the heat and stir in the fresh cilantro.
Serve warm inside dosas or as a side dish.
Don't over-boil the potatoes. They should be tender but still hold their shape to avoid a mushy filling.
Adjust the green chilies to your personal spice preference. You can de-seed them for less heat or omit them entirely.
