Wash the cod, dab dry and cut into cubes. Sprinkle with 3 tablespoons of lemon juice, salt and leave to stand. In the meantime, wash and peel the cucumbers, cut them in half lengthwise and remove the seeds with a spoon.
Cut the cucumbers into rings. Peel and chop onion. Roast mustard seeds in a dry pan and remove. Heat the fat in a large pot and fry the onions until they are glassy. Add cucumbers, braise briefly and season with a little salt and pepper.
Add 1/4 litre of water and cream, bring to the boil and simmer for about 1 minute. Stir in sauce powder and bring to the boil again while stirring. Season to taste with salt, pepper, white wine and remaining lemon juice.
Drain the fish, place it on the vegetables and let it simmer covered for 5-10 minutes. Wash the dill, dab dry and chop finely, except for something to garnish. Carefully fold all but 1 tablespoon into the ragout and place in a bowl.
Sprinkle with remaining dill, mustard seeds and crabs and serve garnished with dill flowers. Rice tastes good with it.