Place the mustard seeds in a large dry pan and cook over medium heat
until the seeds start to pop.
Add the ghee, onion, garlic and ginger, and cook, stirring, until
the onion is soft.
Add the turmeric, chili powder, cumin, garam masala and potato; stir
until the potato is coated.
Add the water, cover and simmer for about 15 to 20 minutes, or until
the potato is just tender, stirring occasionally.
Add the peas and stir until combined; season with salt and pepper to
taste. Simmer, covered, for a further 3 to 5 minutes, or until the
potato is cooked through and the liquid is absorbed.