Clean the cauliflower and parboil the entire head in lightly salted boiling water for 10 minutes. Drain and then separate into fleurettes. Set aside in a warm place (ie bowl with a lid).
While the cauliflower is cooking, grease an ovenproof casserole dish with olive oil. Arrange the fish in the dish, then sprinkle it with the onion and garlic. Season with salt and pepper and the chile powder if using. Pour the wine over the fish and drizzle with the olive oil.
Bake the fish for 20 minutes. About 5 minutes before the fish is due to be done, add the cauliflower to the pan.