Toss the potatoes with 2 tablespoons oil, season and place on a baking sheet. Cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes. Put the tomatoes, cut side up, on the baking sheet, drizzle over 1 tablespoon olive oil, then sprinkle over the vinegar and sugar. Cook for a further 5 minutes until the potatoes are soft and the tomatoes are lightly charred.
Meanwhile, heat a large, ovenproof frying pan until smoking. Season the beef well, then swirl the remaining oil around the pan and add the beef. Cook for 1 minute on each side until browned all over, then transfer the pan to the oven and cook for 12–15 minutes for medium rare. Leave the beef to rest for a couple of minutes, then cut into thick slices.
Stir together the horseradish sauce and crème fraîche. Arrange the beef on a plate along with the tomatoes and potatoes. Serve with the horseradish sauce.