Boil the potatoes until tender, peel and cut into cubes.
In a bowl, mix the plain flour, red chili powder, turmeric and salt. Add some water into this mixture slowly while stirring and make a thin/liquidy paste. This paste will help thicken the gravy of the khara bateta. Set aside.
Heat the oil in a pan, then add the mustard seeds and curry leaves. Fry them a little but keep the heat on low and do not allow them to burn.
Then add the grated mango or tamarind paste and the chopped chilli. Stir-fry quickly then add the tomato puree/paste and cook this for about half a minute.
Next, add in the paste made earlier. Keep stirring on low heat until it thickens and cooks. If it gets too thick, you can easily just add some water.
Once this spicy paste mixture is cooked (takes about 2 minutes on low heat) add the boiled cubed potatoes. Give everything a gentle mix to combine then add some water if the gravy feels too thick.
Allow to simmer for a few minutes to allow the potatoes to absorb all the flavours. Taste and adjust salt and spice. Squeeze in the lemon juice.
Turn off the heat and serve as a side or with accompaniments of your choice.