Remove the skin from your fish, and season both sides with salt and pepper. Start by browning some diced bacon in a large rondeau pot over medium heat, which takes about 6 to 8 minutes.
Remove the bacon and set it aside. Turn the heat to high until it begins to lightly smoke and add in the fish, 2 tablespoons of unsalted butter, and sear it for 2 to 3 minutes per side.
Remove the fish and set it to the side. Add the onions and leeks to the pot and cook over low to medium heat for 15 to 20 minutes or until browned while occasionally stirring.
Pour in the celery and cook for 3 to 4 minutes. Stir in the garlic just until fragrant, which takes 30 to 45 seconds. Deglaze with the cream sherry and cook to au sec or almost gone and absorbed.
Add in the stock, bay leaves, salt, pepper, and potatoes, and bring to a boil. Once boiling, turn the heat to low-medium, and place back in the pre-seared fish.