Preheat the oven to 220C (450F), Gas mark 7. Remove the beef from fridge 30 minutes before you intend to use it to allow it to come back to room temperature. Scrub the beetroots, then trim off the tops. Pat dry with kitchen paper. Place in a roasting tin and drizzle over the olive oil and balsamic vinegar. Season, then cover with tin foil and roast for 20-30 minutes until tender. This will depend on their size, to check if they are done they should be pierced easily with a knife. Leave to cool, then cut each one in half and toss back into the cooking juices to keep them moist.
Heat a large griddle pan until smoking hot. Smash the coriander seeds in a pestle and mortar and then sprinkle on to a board with the rosemary and a quarter of a teaspoon each of salt and pepper on to a board and then roll all over and press the beef over to encrust it. Add to the heated pan and sear for about 6 minutes until well browned and slightly crisp all over, turning regularly. Remove from the heat and leave to rest on a board, uncovered, for at least 20 minutes.
Meanwhile, make the horseradish dressing. Place the horseradish and crème fraiche and chives in a bowl and mix well to combine, then season to taste.
When the beef has rested, slice it as thinly as you can and arrange in an overlapping layer on large plates. Scatter over the roasted beetroots and drizzle some of the beetroot cooking juices on top, then dribble over the horseradish dressing. Dress the watercress with the extra virgin olive oil and lemon juice and scatter all over the finished plates. Garnish with the Parmesan shavings, if using and add a good grinding of black pepper to serve.