For the fish stew, peel and finely chop the onion. Wash zucchini, cut in half lengthwise and slice.
Fry onion in heated butter until translucent. Add zucchini and sauté for 5 minutes. Dust with flour and pour in hot beef broth.
Stir in creme fraiche, add salt and pepper. Stir everything well and steam for 5 minutes.
Dice fish while still frozen (4 cm), sprinkle with lemon juice, mix with zucchini vegetables.
Cover and steam again until fish is cooked. Finally, fold in dill and serve fish stew hot.