In a large pot, melt the butter over medium heat. Add the chopped onion and celery, and sauté until softened, about 5 minutes.

Add the diced potatoes, fish stock, thyme, salt, and pepper. Bring to a boil, then reduce heat and simmer until the potatoes are tender, about 15 minutes.

Stir in the fish pieces and heavy cream. Cook for an additional 5-7 minutes, or until the fish is cooked through.

Ladle the chowder into bowls and garnish with chopped parsley before serving.