Preheat the oven to 450 degrees F (230 degrees C/gas 8).
Wash and scrub the beets, trim the ends, and toss into a large piece of foil with a little oil, balsamic vinegar, and seasoning. Wrap and roast until tender. Cooking time depends on size.
Mix the rosemary salt and pepper on a board. Roll and press the fillet of beef over this, making sure all sticks to the meat. In a very hot, ridged pan, or on a barbecue, sear the meat until brown and slightly crisp on all sides, around 5 minutes. Remove from the pan. Allow it to rest for 5 minutes, then slice it all up as thinly as you can. Lay the slices on a large plate.
After preparing the beef, sprinkle the roasted beetroots randomly (whole, halved or quartered, depending on size) over the sliced meat. Now mix the horseradish and creme fraiche together. It has to be seasoned well, usually needing a little white wine vinegar or lemon juice. Dribble this over the beetroots. Dress some watercress with olive oil and lemon juice. Then scatter this, along with some small slivers of shaved Parmesan, all over the plate and get ready to tuck in!