Wash the potatoes and place them in a large pot with plenty of water and 2 tablespoons of salt. Boil the water and let the potatoes cook for about 20 minutes. Place the potatoes on a baking tray and bake in a preheated oven at 200°C (top/bottom heat) for about 30-50 minutes. They should be soft and the skin should be a little loose.
Grate the Kashkaval cheese finely. Carefully open the potato with a fork and mash the inside. Stir in the cheese, butter and salt.
Stir the horseradish, salt, pepper and chilli into the sour cream. Cut the onion into fine rings and chop the chives finely. Spread the sour cream over the potato and serve the kumpir with smoked salmon, onions and chives.