I found this at the BBC web site. If you like potatoes you will find this divine! I add more horseradish (personal taste). I imagine you can add grated cheese and even a beaten egg before taking to the oven, and get a different kind of texture. Oven temperature should be adjusted; mine, for example, which is quite small, needs lower temperatures. —MrsK
Put cream, milk, garlic and horseradish into a large saucepan; season with salt and pepper and bring to the boil. Reduce heat, add potatoes, stirring so they won't stick together. Cook 10 minutes until tender. Pour into ovenproof dish. (This can be done up to 1 day ahead; bring to room temperature 1 hour before baking.)
Bake at 375 F for 30 minutes. Increase temperature to 400 F and bake 30-35 minutes.