Heat oil in a heavy-bottomed pot. Add kalonji (nigella seeds) and fry on medium heat until they start to pop.
Add crushed ginger and garlic. Fry until the raw smell fades away.
Lower the heat and mix in turmeric, red chili, and coriander powder. Fry for 30 seconds to a minute until aromatic. If the spices stick, splash a bit of water to deglaze the pan.
Turn the heat up and add diced chili, tomatoes, and salt. Cook until the tomatoes start to soften and break down.
Mix in the potatoes well, ensuring they are coated in the masala.
Pour in ½ - 1 cup of water (just enough to cover the potatoes) and bring to a boil. Then lower the heat and simmer uncovered until the potatoes are cooked. Adjust the salt and heat to taste.
Turn off the heat. Gently fold in diced cilantro and drizzle with lemon juice. Cover the pot with a tight lid and let it steam for an additional 5 minutes.
Enjoy!