Peel and finely dice the onion. Peel the celery, rinse and also finely dice.
Heat the olive oil in a saucepan and sweat the onion and celery cubes in it. Pour in the broth and add the peas. Steam the vegetables and fill up with the milk. Salt, pepper and puree the soup.
Dice the potatoes. Divide the fish into bite-sized pieces. Let both simmer carefully in the soup. Cut the tarragon into small pieces. Season the soup with it and with the finely grated lime zest.