Put the vegetable oil in a large heavy-based saucepan and set over a medium-high heat. When hot, add the cumin seeds and green chilli and fry until the cumin seeds turn deep golden brown and become fragrant. Turn the heat down to low and add the cubed potato, cooking gently for 8 minutes until it starts to soften at the edges.
Add the cauliflower florets, ground turmeric, chilli powder, salt and sugar and give everything a good mix together until the cauliflower is fully coated with the spices.
Cover with a lid and cook gently for 15-20 minutes, adding a splash of water only if absolutely necessary and stirring occasionally, until the cauliflower is tender and cooked through. Stir through the chopped coriander leaves, then serve.