Simmer the red pepper for several minutes in salted water and drain. Simmer the Serrano in boiled water until tender. Now place the red pepper, Serrano pepper, potato, garlic, and basil in a bowl or mortar and pound for several minutes until you form a paste.
Drop by drop add in the olive oil for making mayonnaise and beat. Season to taste with salt and pepper. Just before serving loosen up the sauce with 2-3 Tbsp. of hot soup.
Saute the onion in the olive oil until tender in a large saucepan.
Stir in the garlic and tomatoes. Raise the heat to moderate and cook for 5 min. more.
Add the fish stock and water. Then add the parsley sprigs, bay leaf, basil, saffron, pepper, and salt. Simmer for 30-40 min. Strain the soup, pressing out the juices from the ingredients. Correct seasoning.
Drain the soup now back into the pot and place in your larger pieces of fish like the halibut steak. Simmer for 5 min.
Now add the red snapper, scallops, clams, and mussels and cook another 5-7 min. until the mussels and clams open.
Place rounds of the toasted bread in the bottom of each soup bowl. Pour the soup into each soup bowl dividing the fish and shellfish amongst the bowls.
Pass around the rouille, cheese, and parsley.