First prepare your bechamel sauce. Melt half of the butter (7 tbsp) in a pot and add the flour. Also heat up the milk with the heavy cream in another saucepan.
Cook the flour in the butter for 3-4 minutes. It's ready when it’s been foaming for a minute or so.
Remove the flour/butter saucepan from the heat and pour in the hot mixture of milk and heavy cream. Stir quickly.
Put your saucepan back on the stove on low heat. Stir continuously until it thickens. If you used hot milk, it should take no more than 2 minutes.
Remove your bechamel sauce from the heat and let it cool off to room temperature.
Season with salt and ground white pepper.
Beat the remaining half of the butter (7 tbsp, at room temperature) with a mixer for about 2 minutes, until it’s fluffy.
Add the horseradish to your butter and mix it again until it’s fluffy.
Pour in the already cooled off bechamel sauce, and mix the whole horseradish sauce thoroughly with a spatula or wooden spoon.