Heat 3 tbsp oil in a large pan over medium-high heat until shimmering. Add onions and potatoes and cook, stirring constantly, until onions are caramelised and potatoes are golden brown on edges, about 10 minutes. (Reduce heat to medium if the onions darken too quickly).

Reduce heat to medium. Clear the centre of the pan and add remaining tbsp oil, garlic, ginger, chilli (if using), and tomato paste; cook, stirring constantly, until fragrant, about 30 seconds. Add toasted spices and cook, stirring constantly, about 1 minute.

Add cauliflower and cook, stirring constantly, until spices coat florets, about 2 minutes longer. Add tomatoes, water, and chickpeas. Increase heat to medium-high and bring mixture to boil, scraping the bottom of the pan with to loosen browned bits. Cover and reduce heat to medium.

Simmer briskly, stirring occasionally, until vegetables are tender, 10 - 15 minutes. Stir in peas and coconut milk; cook until heated through, about 2 minutes longer. Adjust seasoning with salt and serve with rice.