Cut potatoes into 3/4 inch cubes. Heat the mustard seeds in dry pan until they start to pop. Add ghee, onions, garlic and ginger, stirring until soft. Add turmeric, salt, pepper, chili powder, cumin, garam masala and potatoes. Stir until potatoes are coated.
Add water and simmer, covered 15-20 minutes or until potatoes are just tender; stirring occasionally. Add peas and stir. Simmer, covered, for 3-5 minutes or until potatoes are cooked through and liquid is absorbed. Add cilantro and stir.