Preheat the oven to 200°C/400°F/gas mark 6.
Arrange the cauliflower, potatoes and red onion in a large, deep roasting tray, or over two smaller trays. Drizzle with sunflower oil, then roast in the oven for 20 minutes.
Meanwhile, pour the coconut milk into a jug (pitcher) or bowl, then whisk in the curry paste, tomato purée, mango chutney and chili flakes. Set aside.
Carefully remove the roasting tray from the oven and add the sultanas and chickpeas.
Pour the spiced coconut milk around the vegetables, fully coating the sultanas and chickpeas.
Return the roasting tray to the oven and cook for 30 minutes until the simmering sauce has thickened and the vegetables have softened.
Remove from the oven and season with salt. Drizzle over the lemon juice and scatter with coriander leaves just before serving.