Cook unpeeled potatoes in boiling water until tender. Drain and once cool enough to handle, cut into chunks.
Work ginger and garlic to paste in mortar and pestle or small food processor. Add 3tbsp water and stir.
Heat 1tbsp oil in deep frying pan; add potatoes and onion. Cook over medium heat for 5-6mins until golden, then add turmeric and cook for another 2mins. Add remaining 1tbsp oil and chopped tomatoes, cook for about 5mins until tomatoes are soft, stirring from time to time. Stir in garlic and ginger paste, then add cumin and cayenne pepper; season.
Cook for 2mins, then add peas and 150ml water. Simmer for 4-5mins until thickened. Stir in coriander and serve with plain rice and low-fat plain yoghurt on side.