In a large pot with a lid, bring the stock to a boil. Add the mussels, cover, and steam until the shells open, 4 to 5 minutes.

Remove the mussels from the liquid and allow to cool slightly. Discard any mussels that have not opened. Strain the liquid, cover, and refrigerate until ready to use. Remove the mussel meat from the shells, cover, and refrigerate until ready to use. If you like, you can set aside a few in the shell for garnish.

Rinse out and dry the pot and return it to the stove. Add the bacon and cook over medium heat until crisp. Remove the bacon, leaving the fat in the pot, and dry on a paper towel.

Add the butter to the bacon fat and heat until foamy. Add the diced onions, fennel, and celery; cook until soft and translucent, 8 to 10 minutes, stirring occasionally. Season with sea salt.

Meanwhile, in a medium-size saucepan, place the milk and cream and cook over medium heat, but do not allow to boil.

Stir the flour into the vegetables and cook, stirring, until thickened— this will create a liaison and prevent the chunky bits of vegetable and seafood from sinking to the bottom of your bowl later.

Pour the hot milk mixture slowly into the vegetables, stirring constantly until well blended and smooth. Bring back to a simmer, stirring frequently.

Stir in the potatoes, bacon, and reserved cooking liquid from the mussels. Return to a simmer and cook until the potatoes are tender, 15 to 20 minutes.

Add the cooked mussels, chopped oyster meat, and cubed fish and cook for 4 to 5 minutes, stirring occasionally. Season with salt and pepper and, if you like, a few dashes of hot sauce and/or Worcestershire sauce.