Scrub and wash the potatoes, then cook them in a pan of lightly salted water. Drain and set aside.
Peel the ginger and garlic, then grate them. Peel the onion, then thinly slice. Roughly chop the tomatoes.
When the potatoes are cool, cut them into bite-sized pieces.
Heat the oil in large frying pan and fry the onion for 5 minutes, then add the potatoes and cook for other 5 minutes. Add the grated ginger and garlic, stir for a minute to coat, then add the turmeric, chilli and cumin. Stir to coat, then add the tomatoes and a glass of water, or the tinned plum tomatoes. Season with salt & pepper, then bring to boil.
Add the green peas and more water if the sauce is too thick. Cook for 5 minutes under cover.
Serve with the chopped coriander scattered on top.