Put all the cauliflower florets in a large bowl. Combine the lemon juice, turmeric and ginger in a small bowl, then pour over the cauliflower. Add the salt, cayenne, ground spices and fresh coriander and mix well, using a plastic glove if preferred. Set aside for 2 hours, tossing now and then. Preheat the oven to 220°C/gas mark 7. Put the oil in a small frying pan and set over a medium-high heat. When hot, add the cumin seeds, let them sizzle for a few seconds, then pour the spiced oil over the cauliflower. Toss well. Spread out the cauliflower in a single layer in a large roasting tin. Place in the oven for 15 minutes, then turn the pieces and roast for another 10–15 minutes, or until lightly browned and cooked through.