Heat the vegetable oil in a saucepan over a medium heat. When hot, add the onions and fry for 2-3 minutes, or until softened.
Add the ginger, mustard seeds, curry leaves, ground turmeric, ground fenugreek, whole green chillies, chilli powder and salt. (Caution: when heated, the mustard seeds will start to pop.) Stir the mixture well and cook for two minutes, or until the seeds start to pop and the spices smell fragrant.
Add the potatoes and stir well, then add the tomatoes, cauliflower, lemon juice, sugar and water and bring the mixture to the boil. Reduce the heat and simmer the mixture for 25-30 minutes, or until the potatoes have cooked through and the mixture has thickened.
Add the potatoes and stir well, then add the tomatoes, cauliflower, lemon juice, sugar and water and bring the mixture to the boil. Reduce the heat and simmer the mixture for 25-30 minutes, or until the potatoes have cooked through and the mixture has thickened.