Place the butter into a warm deep saucepan, add the garlic and cook for 20 seconds. Combine in the leeks and onions and cook gently for 5 minutes until soft but no coloured.
Add in the stock, potatoes, celery and thyme sprig. Bring to a boil then turn down and simmer for 20 minutes or until the potatoes and celery are tender.
Remove the thyme sprig, flake the salmon and add it to the broth along with the cream, dill and seasoning. Stir and simmer for several minutes before serving.