The day before, wrap the beet individually in tin foil with a little salt and caraway seeds. Pour the sea salt into a shallow saucepan, place the beet on top and cook in a hot stove at 180 degrees for 60 to 90 minutes.
When the beet is soft, wrap it from the foil and peel it off. Cut the peeled beet into 2 mm thick slices. Prepare a marinade from beef broth, 2 vinegars, oil and juice of one lemon.
Chop the caraway seeds and add. Season with cayenne pepper, salt, pepper and a pinch of sugar. Pour in the onion halves, garlic and beet slices while they are still lukewarm and leave to marinate at room temperature for a day.
A day later, lift the onions and garlic out of the marinade. Pour in the gelatin soaked, squeezed and dissolved in a tiny bit of warmed marinade, stir well and season repeatedly. The beet must not be too cooled, otherwise the gelatine will attract too quickly and lumps will form.
For the mousse, stir together sour cream, crème fraîche and horseradish. Season with pepper, juice of one lemon, salt, cayenne pepper and sugar and also leave for one night. The following day, strain through a fine sieve. Add the dissolved gelatine and finally stir in the whipped cream.
Cover a baking dish with cling film and place in a baking dish filled with ice cubes.