Peel, wash and roughly grate the potatoes. Put potatoes in boiling salted water and simmer for 3 minutes. Pour into a sieve, quench and drain well. In the meantime wash dill, shake dry and chop finely.
Cucumbers wash, peel, cut off ends. Cut each cucumber into 3 pieces. Remove the seeds carefully with a tablespoon. Put the cucumbers in an ovenproof dish. Cut fish into small cubes. For the filling mix potatoes, dill and fish, season with salt and pepper.
Spread the filling evenly into the cucumbers. Dice butter. Mix sauce powder and 125 ml water in a small pot. Bring to the boil briefly while stirring. Remove from the heat and fold in the butter until it has melted.
Pour over the filled cucumbers and bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 30-35 minutes. Remove from the oven and garnish with dill flowers.