Peel and chop the onions into a ¼ inch dice. Chop the celery into similar-sized pieces. Mince the garlic. Peel and chop the potatoes into ¼ inch cubes, then soak them in a bowl of water until ready to use. Chop the cod into ¼ inch cubes.

Set a large 6 to 8-quart pot over medium heat. Add the butter and flour to the pot. Whisk as the butter melts to create a foamy roux for thickening. Whisk the roux for 2 to 3 minutes until it reaches a golden color.

Stir in the chopped onions, celery, and garlic. Sauté for 2-3 minutes, then drain the potatoes and add them to the pot. Stir and sauté the vegetables for another 8-10 minutes to soften the aromatics and partially cook the potatoes, stirring every 2-3 minutes.

Add the diced cod, seafood stock, clam juice, cooking sherry, Old Bay seasoning, 1 teaspoon of salt, and ½ teaspoon of black pepper. Stir well and bring to a simmer. Simmer the chowder for 8-10 minutes to thicken the soup base and cook the fish.

Once the potatoes are soft and the fish is cooked through, stir in the heavy cream and parsley. Taste, then add salt and pepper as desired.

Serve warm as-is or with chopped scallions and/or oyster crackers for a garnish.