Place the potatoes in a large pan. Cover with cold water and bring to a boil over high heat. Cook for about 10 minutes or until potatoes are just tender. Drain thoroughly.
Heat the oil in a large frying pan. Add the onions and cook over low heat until very soft. Increase the heat and cook the onions until golden brown.
Add mustard and cumin seeds and cook for a few minutes until they start to make popping sounds.
Stir in the garlic, ginger, cumin powder, garam masala, chili powder, coriander, turmeric, salt, and black pepper.
Add in tomatoes and stir, letting the liquid from the tomatoes seep into the curry
Cook over a medium to low heat until the mixture becomes thick. Season to taste and then stir in the potatoes. Simmer for 1 minute and then remove from the heat and leave to cool.
Enjoy!!