In a heavy saucepan, melt butter with the olive oil and add the onion, celery and carrot. Cook over low heat for 5 minutes, stirring frequently so it doesn't brown, then add leeks. Add salt and pepper to taste and a little dried thyme. Cover and cook 10-12 minutes, stirring frequently.
When the leeks are tender, add the potatoes and saute for a minute. Add the fish stock, wine and water. Bring to a boil, reduce heat and simmer 15-20 minutes, stirring occasionally, until tender.
In a bowl, combine the flour with half the milk with a whisk. Add to the saucepan, simmer and let thicken for several minutes.
Add the remaining milk and salmon pieces and simmer 5-8 minutes, until salmon is done. Serve hot, garnished with a little dill.