Soak the channa overnight, then boil them with very little salt until soft (too much salt will make them remain hard). This can take up to half an hour in a pressure cooker, longer in a regular pan. Make sure they are boiled until VERY tender, drain and retain half a cup of the water, discard the rest.
You can add a pinch or two of baking soda when boiling these to soften them faster if you want!
Heat the oil, add the mustard seeds and let them splutter, then add the limro/curry leaves followed by the green chillies. Lower heat and add the garlic, stir for a while then add the grated mango and tomato paste. Cook this for a few minutes, then add the boiled cubed potatoes and the chilli powder, turmeric and cumin powder. Let this cook for a bit stirring all the time, you can add a bit of water if it starts getting stuck. Then add the channa.
Stir together and cook for a few more minutes until the flavorful masala covers everything, then add the retained half cup of channa water and enough water to almost completely cover the channa in the pan. Adjust salt and cook for a few minutes until it comes to a soft boil. Then stir in your corn flour or flour mixture if you are using it followed by the tamarind pulp, bring to a boil until the water thickens slightly. Adjust salt if needed, and if you like you can squeeze a little lemon, though sometimes the mango is enough.
Another way to thicken the sauce without using the flour is by gently smashing some of the potatoes and channa by pressing them with the flat side of a spoon and then stirring them back in. The starch released from the potatoes will help thicken the sauce.
Turn off stove and serve hot with some crispy daal bhajias, cheuro or potato/cassava crisps and a spicy coconut chutney.