Sprinkle with flour. Add butter to melt. Stir and cook for a few minutes.
Slowly add the fish stock and milk, and bring to a simmer.
Add the smoked fish, potatoes, and leeks. Simmer on low stiring often until the leeks are cooked.
Finish by stirring in the cream and chives. Season with salt and pepper to taste.
Notes
If you'd like the soup to be thinner add more liquid like the chicken stock. If you want it to be thicker, you can add some more roux. Allow it to simmer and mingle. It's even better the second day.