Wash and scrub mussels and clams (observe all safety rules when dealing with seafood).
Bring 2 cups of water to a boil in large pot. Add the mussels and cover the pot. Steam mussels for 6-8 minutes (when the mussels open)... any mussels that do not open must be thrown away. Remove from heat.
Cut the fish into 1" thick slices.
Heat oil in a large stew pot. Add the garlic, onions, tomatoes, fennel, salt, and pepper. Add the broth from steaming the mussels and butter to the pot.
Add remaining hot water and clam juice. Bring to a boil.
Add clams and fish and continue to boil for 6–8 minutes.
Add mussels and shrimp. Boil for a final 5 minutes.
To serve: Scoop out a generous portion of seafood and ladle the broth over it. Serve with a large chunk of your favorite rustic style bread.