Wash the potatoes and cook in boiling salted water for about 20 minutes. Then rinse with cold water and peel. In the meantime wash salmon and dab dry. Cut into 2-3 cm cubes.
Sprinkle with lime juice and salt a little. Wash the dill, dab dry and chop. Peel and finely chop the onion. Melt the fat. Steam onion in it until transparent. Add flour and sauté. Deglaze with stock and cream.
Bring to the boil while stirring continuously. Season to taste with salt, pepper and horseradish. Stir in dill. Fill potatoes and salmon cubes into a gratin dish. Spread the sauce on top. Bake in the preheated oven (electric: 225 °C/ gas: level 4) for about 12 minutes.
Serve garnished with lime wheels and bouquet of herbs.