Defrost deep-frozen shrimps at room temperature on a sieve for about 1 hour and drain. Wash the cucumbers, drain, peel and cut them in half lengthwise. Core cucumbers with a spoon and cut the halves into thick slices. Peel and chop onion. Wash, drain and peel the prawns. Cut the prawns lengthwise at the back and remove the intestines.
Heat the oil in a pan and fry the prawns for about 2 minutes, turning them over. Season with salt and pepper, remove and keep warm. Put butter and onion into the hot pan and fry briefly. Add cucumbers, braise while stirring, season with salt and pepper. Add stock and pernot, bring to the boil, cover and simmer for 6-7 minutes.
Wash the salmon, dab dry and cut into cubes. Whip cream until semi-stiff and mix half of the cream with starch and mustard. Add the salmon to the gherkins, bring to the boil briefly, cover and allow to simmer for 2-3 minutes. Wash the dill, shake dry and pluck the dill flags from the stalks. Cut dill finely. Season the cucumber pan with salt, pepper, lemon juice and sugar. Place the prawns in the pan and spread the remaining cream in patches on top. Sprinkle with dill. Serve with fresh bread or baguette.