Put the potatoes in a large pan, cover with cold water and bring to the boil. Cook for 15 minutes until tender. Drain.
Using a pestle and mortar, roughly grind the cumin seeds. Heat the oil in a large frying pan, then add the cumin, ginger and garlic and cook for 30 seconds. Add the cooked potatoes, peas and 100ml water and simmer, covered, for 5 minutes.
Stir through the creamed coconut, lemon juice and coriander. Season and serve.