Heat the mustard seeds in a dry pan until they start to pop.
Add the oil or ghee, onion, garlic and ginger and cook, stirring until onion is soft.
Add the turmeric, chilli powder, cumin, garam masala and potato,
Season with salt and black pepper.
Stir until potato is coated with the mixture.
add 125 ml of water and simmer, covered for about 15 - 20 minutes, or until the potato is tender.
Stir occasionally to stop the curry sticking to the bottom of the pan.
Add the peas and stir until well combined.
Simmer, covered, for 3 - 5 minutes, or until the potatoes are cooked and all the liquid is absorbed.
Stir in the mint and season to taste.