Clean, wash and halve the beans. Put them into boiling salted water and cook for about 12 minutes. Then pour into a sieve and drain. In the meantime peel and finely dice the onion. Wash the fish, dab dry, cut into slices, sprinkle with lemon juice and salt. Heat oil in a pot, fry onion in it until transparent, dust with flour, sauté and gradually add stock and cream while stirring. Let it boil up.
Add mustard and stir in. Season to taste with salt, pepper and sugar. Put the beans into an ovenproof dish, put the fish in it and spread the sauce evenly over it. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15-20 minutes. Remove from the oven and serve immediately garnished with lemon and dill. Delicious with baguette