Heat 100g of the butter and fry the cauliflower until it starts to colour. Add the onion and continue to fry, stirring regularly to cook evenly. Season with salt and pepper and cook until the cauli is starting to break up and the edges are brown.
When the cauli is nearly ready, mix the spices and sprinkle a little of the spice mixture evenly on each side of the fish. Heat the remaining butter in a frying pan and fry the fish until cooked through, turning only once.
Serve the fish with the cauliflower and a squeeze of lemon.