Generously season the beef fillet with salt and pepper. In a pestle and mortar bash up about a quarter of the rosemary with a clove of garlic to make a paste. Loosen with 5 tablespoons of olive oil and then rub this all over the beef. Tie the beef up with 4 pieces of string then poke the remaining rosemary sprigs under the string. This way the beef is almost protected by the herb during the cooking and it will also give great flavour.
Preheat the oven to 250ºC/450ºF/gas 8. Parboil your sliced potatoes in boiling salted water for around 5 minutes. Drain in a colander, and transfer to a bowl with just enough olive oil to coat them. Season well. I like to make the potato cake in a round greased or non-stick cake tin, but you can use a non-stick frying pan with a metal handle if you have one. Or you can even make small individual ones. Place half the potatoes into the tin or pan, smearing the creamed horseradish over the top. Place the rest of the potatoes on top then pat down and put to one side.
Brown off the beef in a snug-fitting roasting tray until all sides are coloured. Add the garlic cloves to the tray, place the beef on top of them and put in the oven with the potatoes on a shelf below. Cook for 20 minutes, then turn the beef over, baste it and add the red wine and butter to the tray. Remove the potato dish then carefully place a clean tea towel over it and push down to compact the spuds into a nice tight cake. Put the potatoes back into the oven for another 15–20 minutes.
I serve the beef cooked medium, but you can cook it more or less to your preference. Remove the beef from the oven and while it is resting continue browning the potato cake for 5 minutes if it needs it. When you're ready to serve, remove the string and the rosemary sprigs from the beef and carve it into nice slices.
Turn the potato cake out on to a board, or just scoop it out of the pan with a spoon if it's stuck and divide between your 6 serving plates beside the meat. Save any juices from the rested meat and return them to the tray, where the red wine and butter and all the goodness from the meat will have made a very simple but tasty cooking sauce. Finish by mushing up the garlic cloves, then pass the sauce through a sieve on to the meat. Lovely served with some dressed watercress.
Try this: You could do the same recipe with a pork loin or venison.