Preheat the oven to 180°C.
Steam the cauliflower for 10 minutes, or until tender. Let cool slightly, then transfer to a blender. Add the mustard and a pinch of salt and process until smooth. Transfer to an ovenproof dish, sprinkle the parmesan on top and place in the oven for 15 minutes.
Pour the milk into a deep frypan with a lid. Add the salmon, prawns and broccolini. Slowly bring to the boil, then reduce heat to a simmer, cover the pan and cook for 5 minutes. Remove from the heat, pour the milk through a sieve and reserve the liquid to make a sauce.
Cover the cooked fish, and broccolini with foil and set aside to stay warm.
In the frypan, heat the olive oil over medium heat, stir in the flour and cook for 1 minute. Whisk the reserved milk into the flour mixture a little at a time to make a smooth sauce. Stir in the lemon juice and corn, season with some cracked black pepper and cook for another minute.
Pour the sauce over the fish and serve with the cauliflower mash.