Peel, wash and halve the potatoes and cook in boiling salted water for about 20 minutes. Clean, wash and slice the leek. Blanch in boiling salted water. Heat oil in a pan and fry fish fingers in portions for about 8 minutes until crispy on all sides. Heat milk and butter.
Drain the potatoes, add the milk-butter mixture and horseradish and mash the potatoes into a puree. Season to taste with salt and nutmeg. Put the mashed potatoes, leek and fish fingers into a greased casserole dish. Sprinkle with cheese. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes