Place a skillet over low heat and add the coriander seeds, cumin seeds, fenugreek seeds and black mustard seeds. Toast for 15 seconds until aromatic and then crush in a mortar and pestle.
Place a large pot over medium heat and add the oil. When it is hot, add ginger, garlic and onions, and cook until very soft and beginning to turn golden, about 4 minutes. Add the curry leaves, garam masala and turmeric and continue to cook for 10 seconds. Add the tomato paste and stir well. Add the toasted spices, tomatoes, salt and pepper, buttermilk, potatoes and 1 cup water, and bring to a boil. Reduce the heat and let simmer until the potatoes are tender, 45 minutes to 1 hour. Taste for seasoning. Add salt and green chilies to taste.