Heat the ghee and mustard oil in the kadai pan. Add the cumin, black mustard seeds, and ginger to ghee and stir fry a few seconds 'til seeds pop and sizzle.
Add the diced tomato and cook over low heat until soft, stirring frequently.
Add the coriander, turmeric, chili, salt and pepper. Stir to mix. Add the water.
Add the cut green beans and cubed potatoes. Mix and stir frequently. Cover and cook until vegetables are done, about 15 minutes, stirring frequently, and adding more water if necessary.
Remove from heat; stir in the fresh coriander (cilantro) leaves and serve.
Notes
If you don't have the mustard oil, you can omit it, but it does give a nice and unique flavor. I'm pretty sure mustard oil is not approved for eating outside of Asian countries like India where it is used almost exclusively. So what you may find is mustard oil that is "not for consumption, for external use only", which must be stated by law. All I'm going to say is, a lot of people use mustard oil for cooking all around the world.You decide.