Pat cod, shrimp and scallops dry and refrigerate while preparing the base. Melt butter in a soup pot over medium heat and cook fennel and onion for 8 minutes until soft.
Add potatoes, seafood stock, salt and pepper. Simmer gently for 15 to 18 minutes until potatoes are tender.
Stir in milk and cream. Whisk cornstarch with cold water, add to the soup and simmer gently for 2 minutes until lightly thickened.
Add cod and shrimp and cook at a very gentle simmer for 3 minutes without vigorous stirring. Fold in scallops and cook for another 2 to 4 minutes until cod and scallops reach 145°F and shrimp are opaque throughout.
Ladle carefully among 6 bowls, dividing the cod, shrimp, scallops and potato pieces evenly. Serve immediately.