Wash the potatoes and cook in boiling water for 15-20 minutes. Drain, rinse with cold water and peel. Leave to cool. Cut cucumbers into small pieces. Wash chives, dab dry and cut into fine rolls.
Wash the fish, dab dry and cut into pieces. Sprinkle fish with lemon juice and season with salt. Cut bacon into pieces. Heat the oil in a large pan, fry the bacon and potatoes for about 5 minutes, turning them over until everything is lightly browned.
Turn the fish in flour, add to the potatoes and fry. Turn from time to time. Whisk the stock with mustard, pour into the pan and heat briefly until the liquid is almost absorbed. Season to taste with salt and pepper.
Mix in the cucumber and sprinkle with chives. Serve garnished with dill flowers.