Rinse the fish with cold water, pat dry and cut into pieces approx. 2 cm in size. Peel and halve the cucumbers and scrape out the core with a teaspoon. Cut the cucumber into pieces approx. 1 cm wide.
Heat 1 tablespoon butter in a coated pan. Season the fish pieces with salt and pepper and fry them in the butter for 2 - 3 minutes, turn them once in between, take them out and put them in the oven at 70 ° C.
Heat the remaining butter in the pan and stew the cucumber pieces for 2 - 3 minutes. Dust the flour over it and sauté briefly. Pour in the broth, stir in the crème fraîche and mustard and bring to the boil while stirring. Cover and simmer over medium heat for 8 minutes.
In the meantime, peel the prawns, devein and fry them pink on both sides in another pan in a little olive oil.
Add to the cucumber with the fish and season to taste with salt, pepper and lemon juice. Wash the dill, spin dry well and chop finely. Sprinkle over the pan and serve immediately with boiled potatoes.