Sauté the vegetables: In a large pot, melt the butter over medium heat. Add the onion and celery and cook until softened, about 5 minutes.
Add cauliflower and broth: Add the chopped cauliflower and fish broth. Bring to a boil, then reduce the heat and let it simmer until the cauliflower is tender, about 15-20 minutes.
Blend the soup: Using an immersion blender, puree the soup until smooth.
Add seafood and cream: Stir in the heavy cream and mixed seafood. Cook for 5-7 minutes, or until the seafood is cooked through.
Season and garnish: Season with salt and pepper to taste. Garnish with fresh parsley and serve hot.