Step 1In the bowl of a blender, place the egg. With the motor running, add the oil in a slow stream until it is emulsified (it will thicken like a mayonnaise). Blend in the lemon juice, then set aside. (If the emulsion happens to "break" while it is blended, simply pour out the mixture and add a fresh egg white to the blender container. Add the mixture in a slow stream to fix the emulsion.)
Step 2In a large, heavy-bottomed soup pot, add the fish stock and bring to a boil over high heat. Add the potatoes, cover and simmer for 10 minutes, until somewhat softened but not cooked through. Add the peas and cook an additional 5 minutes.
Step 3Add the chunks of fish, ham, shrimp and sherry. Bring the soup to a boil, then reduce to a gentle simmer.
Step 4With the blender running, ladle about 1 cup of the hot broth from the soup into the emulsion in the blender and blend until smooth. Remove the soup from the heat and whisk the emulsion into the soup. Add salt and pepper to taste. Serve immediately. The soup can be reheated, but do not boil. This makes a generous 2 1/2 quarts of soup.