Place cauliflower in a steamer, and cook for 10-15 minutes or until tender.
Process cauliflower in a blender with butter until smooth. Transfer to a bowl and incorporate the cheese through it.
In a large saucepan over medium heat, melt butter. Add the spring onions and cook until soft. Stir in flour, cook for 1-2 minutes then slowly whisk in some chicken stock and sour cream. Bring sauce to a simmer, once it has thickened take the pan off the heat. Add the smoked salmon, prawns, peas, and the leftover fish. Season to taste.
Transfer mixture to a baking dish, top with the cauliflower mash spreading it evenly. Top with a sprinkle of parmesan cheese.
Place in the oven and bake for 10-15 minutes until the tops are golden.