Heat oil in pan over medium heat. Throw in mustard seeds and cumin. Cook for two minutes.

Add coriander, turmeric, cayenne, and cook for another minute

Add the rest (potatoes, ginger and garlic paste, water, chili powder) and stir so everything is coated well. Stir every 5 minutes and let it cook for 45 minutes. Put it on low heat (2 or 3 setting) to prevent burning.