Wash, clean and slice the celery. Wash, peel and slice the potatoes. Wash and quarter the tomatoes, cutting out the stalks. Remove seeds from quarters and cut into slices. Steam celery and potatoes for 10 minutes in the broth.
Wash and clean spring onions and cut them diagonally into rings. Add tomatoes and spring onions to the vegetables. Season with salt and pepper. Bring crème fraîche and soup paste to the boil, season with salt and pepper. Mix half of the sauce with the vegetables. Pour into a flat casserole dish (approx. 1.5 litres capacity). Rinse fish, dab dry and cut into 2 cm cubes. Salt the fish cubes, turn in flour and fry briefly in the oil. Spread on the vegetables and sprinkle with mustard seeds. Add the rest of the sauce.
Pour into a flat casserole dish (approx. 1.5 litres capacity). Rinse fish, dab dry and cut into 2 cm cubes. Salt the fish cubes, turn in flour and fry briefly in the oil. Spread on the vegetables and sprinkle with mustard seeds. Add the rest of the sauce. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for 15-20 minutes. Garnish with chervil