Put the cauliflower in a roasting tray and toss together with 2 tablespoons oil, the cumin seeds and lemon. Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 20–25 minutes until soft and lightly charred.
Meanwhile, heat the remaining oil in a pan, add the onion and cook for 5 minutes until softened. Stir in the curry paste and cook for 1 minute. Pour over the tomatoes and leave to simmer for 15 minutes. Season to taste.
Heat a dry frying pan and cook the cashew nuts until golden, then set aside to cool. Squeeze a couple of lemon wedges over the roasted cauliflower and pile on a plate. Spoon over the tomato sauce, scatter with the cashew nuts and fresh coriander and serve with plain boiled rice.