Peel the potatoes, wash and dab dry. Place one potato on each tablespoon and use a sharp knife to cut closely together up to the edge of the spoon. Melt the fat and spread half of it on an ovenproof dish. Place the potatoes with the incisions facing upwards, brush with a little fat and season with salt. Bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for approx. 45 minutes.
Brush with the remaining fat in between. Sprinkle with thyme and cheese about 20 minutes before the end of the cooking time. In the meantime, whip cream until semi-solid. Mix mayonnaise and horseradish, fold in cream. Arrange some potatoes each with 2 slices of salmon and horseradish cream on a plate