Heat the oil in a large pan. Add the onion and leek and cook gently for 5min, until softened. Pour in the milk, add the potato and bring to the boil. Reduce the heat and simmer for 5 min, then stir in the flour.
Add the smoked haddock. Simmer for 7min, or until the fish is just tender. Taste and season. To serve, sprinkle the chowder with thyme.