Heat the oil and cook the leek, garlic and bay leaf for 5 minutes. Add the flour and cook out for two minutes, then add the potatoes and mushrooms.
Add the liquid and tarragon and gently bring to the boil, immediately reducing to a simmer. Cook until the potatoes are cooked through (about 20 mins).
Add the mustard, crème fraiche and seasoning to the soup and stir well before adding the fish. Continue to cook for about six minutes more, or until the fish flakes apart when pushed.
Turn the heat off and add the lemon juice and zest. Serve topped with any reserved tarragon.
Tip: Dill would work well as an alternative herb to tarragon in this recipe