Cut cod into pieces, reserving any scraps. Peel and devein prawns, reserving any scraps.
Place cod and prawn scraps, quartered onion, carrot, celery, salt, and pepper in a large saucepan; cover with water. Bring to a boil; reduce heat and simmer until fish stock is fragrant, 15 to 20 minutes. Strain and reserve fish stock.
Heat olive oil in a saucepan over medium heat; cook and stir sliced onion, red chile pepper, and minced garlic until softened, 5 to 10 minutes. Pour in wine; cook for 5 minutes. Add tomatoes; bring to a boil. Add cod, prawns, clams, and mussels; pour in 1 cup fish stock. Simmer until fish is cooked through and clams and mussels have opened, 15 to 20 minutes.
Toast bread in a toaster. Rub garlic halves on to 1 side of toasted bread.
Place a piece of toasted bread in the bottom of each serving bowl. Ladle soup over toasted bread and top with parsley.