Put a large, heavy cast iron or ovenproof skillet over medium-high heat. Add a 2-count drizzle of oil and get it smoking hot. Put the tenderloin in the skillet and sear the outside of the meat all over, 5 to 8 minutes total. Put it in the oven and roast 15 to 20 minutes until rare (the internal temperature of the meat will be 120 degrees F). Take the meat out of the pan, wrap it in plastic, and refrigerate until it gets until good and cold, 1 to 2 hours.
While you wait for the beef to chill, stir the horseradish, sour cream, lemon juice and oil together in a bowl. Season with salt and pepper.
When the beef is cold, take a long, very sharp knife, and cut the meat into slices as thin as you can make them. Lay them out on a platter (they'll overlap – that's fine). Now drizzle the horseradish sauce all over the meat. Hand crush the tomatoes in a bowl and add olive oil to make them saucy; season that with salt and pepper too so that it tastes really good. Drizzle the tomato mixture over the top and garnish the whole thing with watercress.