Heat the oil in a heavy-based saucepan, add the onion and cook over a medium heat, stirring occasionally, for about 10 minutes, or until golden. Alternatively, add the cumin seeds and cook, stirring frequently, until they sizzle.
Add the vegetables, chilli powder, coriander, turmeric and salt to taste and cook, stirring constantly, for 2–3 minutes.
Stir in the tomatoes or the lemon juice. If a dry vegetable curry is preferred, add only a little water, cover and cook gently for 10–12 minutes until dry. For a more moist curry, stir in the measurement water, cover and simmer for 5–6 minutes until the vegetables are tender.
Serve as a main dish with naan, chappatis or basmati rice, or on its own as a side dish.