Add cooking oil to a pot and let the oil heat on medium flame. When oil is hot, add cumin seeds, slit green chillies, curry leaves and fry for a second. Be careful when frying green chillies and curry leaves as moisture from them causes the oil to pop and splatter.
Now add finely chopped onion, a pinch of salt and fry till onions turn light brown in color. Do not burn the onion. In meantime, puree the tomatoes and keep aside.
When onion is almost done, add ginger and garlic paste and, fry for 2 mins.
Add pureed tomatoes to the pan, mix everything well and let cook on medium heat till oil separtes ( it takes like 4 mins ). This step is important, not well cooked tomatoes will give raw taste to the curry.
When tomato puree is cooked, add ground turmeric, corainder powder, red chilli powder, salt and fry for a minute.
Add diced potatoes, peas, water to the pot, mix everything together and cook on medium heat with liid covered. Cook till potatoes are well cooked. Add 1/2 cup more water if required to cook potatoes.
When potatoes are almost done, add dried mango powder, chopped cilantro and cook for 2-3 mins on low heat.
Turn off the flame, squeeze half a lime juice on top of the curry and serve. Goes well with rice or flat bread (roti).
Notes
Can store in an air-tight container in the refrigerator for up to 2 days. Reheat with 2-3 tbsp of water before serve.