Peel the potatoes. Cut the potatoes into half lengthwise. Now cut them into 3 mm slices, much like we cut for machher jhol. Then cut the tomatoes into half and slice them as well.
In a kadai or a frying pan, take a tbsp of mustard oil and when it heats, add the nigella seeds and one chopped green chili. Stir it for a few seconds and add the potatoes. Add turmeric powder, red chili powder and salt and stir fry on high heat for half a minute. Then reduce the flame to medium and continue frying the potatoes for a couple of minutes more.
At this point, add the tomato slices, some sugar and continue mixing. Add more salt if required but remember that the tomatoes will reduce in quantity at a later stage.
Reduce the flame and cover and cook for five to eight minutes. Remove the cover and stir to make sure nothing sticks to the pan. In case you feel that the vegetables will stick to the pan, then sprinkle some water. Stir and cover and cook till the potatoes have become soft.
Take a bowl with the mustard paste, poppy seed paste and a couple of tbsp of water and add this to the vegetables when the potato has become soft.
Mix everything thoroughly and cook for 3 to 4 miutes over high heat till the raw smell of the poppy seed paste goes off. Add a little bit more of water, if you feel everything is becomes too dry.
Once you have achieved the desired consistency, garnish with chopped coriander leaves, more chopped green chilies and a drizzle of mustard oil.
Turn off the heat and serve with steamed rice.