Cook the potatoes whole in boiling water for 15–20 minutes until tender. Drain and leave until cool enough to handle, then peel off the skins and dice the flesh.
Heat the oil in the drained and dried potato pan, add the onion and fry for 3 minutes, then add the mustard seeds, cumin seeds and green chillies and cook for 2 minutes. Mix in the crushed and chopped coriander, turmeric, then the potatoes, peas and plenty of salt and pepper.
Make the pastry. Add the flour, a little salt and the butter or ghee to a mixing bowl, rub in the butter with your fingertips until you have fine crumbs, then mix in enough of the water to form a soft but not sticky dough. Knead well until smooth and elastic. Cut into 9 pieces and shape into balls. Roll out to 12 cm (5 inch) circles.
Cut each circle in half, brush the edges with water, then shape into a cone by folding one point to the centre of the curved top; do the same with the other point. Spoon the filling into the cone, then press the curved edges together to seal. Repeat to make 18 samosas.
Half fill a large saucepan with oil, heat to 180°C (350°F) on a sugar thermometer or until the oil bubbles when a samosa is dropped into the oil. Cook the samosas in batches of 3 or 4 for 3–4 minutes until golden brown, then lift out and transfer to a plate lined with kitchen paper. Serve warm with mango chutney or cucumber raita.