1. In a large pot, melt the butter over medium heat. Add the onion, celery, and carrots, and cook until softened, about 5 minutes.
2. Stir in the flour and cook for another 2 minutes.
3. Gradually add the fish stock or water, stirring constantly to avoid lumps. Add the potatoes, thyme, bay leaf, salt, and pepper. Bring to a boil, then reduce the heat and simmer for 15 minutes, or until the potatoes are tender.
4. Add the salmon chunks and cook for another 5-7 minutes, until the salmon is cooked through.
5. Stir in the heavy cream and heat through, but do not boil. Remove the bay leaf.
6. Serve hot, garnished with fresh parsley.