Cook the potatoes in a large pan of boiling water for about 10 minutes, or until half done.
Drain, allow to cool, then peel off their skins. Cut the potatoes into cubes.
Cook the peas in another pan of boiling water for 5-7 minutes, or until soft, then drain and refresh immediately in cold water so they do not lose their colour. Set aside.
Heat the ghee or oil in a heavy-based pan over medium heat, add the mustard seeds and cumin seeds, and stir-fry for 1 minute, or until they start to splutter.
Add the turmeric, then add the potatoes and tomatoes, if using. Season with salt and fry for about 5-10 minutes, or until done.
Add the peas, chillies, and coriander leaves to the pan, mix well and serve hot.