In a large pot, melt butter over medium heat. Add onion, celery, and carrots. Cook until softened, about 5 minutes.
Stir in flour and cook for another 2 minutes.
Gradually add fish stock or chicken broth, stirring constantly.
Add potatoes, thyme, bay leaf, salt, and pepper. Bring to a boil, then reduce heat and simmer for 15 minutes.
Add salmon chunks and cook for another 10 minutes, until salmon is cooked through.
Stir in heavy cream and cook for an additional 5 minutes.
Remove bay leaf, garnish with fresh parsley, and serve hot.