Heat the oil in a wok or large frying pan until hazy. Add the fenugreek seeds; then 10 seconds or so later, add the mustard seeds. When they stop crackling, add the cuminand crushed coriander seeds and stir for 1⁄2 minute. Add the green chillies and sauté for a further 1⁄2 minute, then add the spring onions and cook for a further 2 minutes.
Toss in the apples and potatoes and add the tomato, cooking for a further 2–3 minutes and tossing occasionally. As soon as the apples soften slightly check the seasoning and, if required, add a squeeze of lemon juice and a pinch of sugar and salt, to taste.
Sprinkle in the coriander, toss all the ingredients together and remove from the heat. Serve immediately.