Barbecued fillet of beef with horseradish butter

Method

1. Trim and discard any fat and silvery sinew from the fillet. If your fillet is thinner at one end, neatly fold the end under itself to form an even thickness, then tie kitchen string along the fillet at 3cm intervals to keep it neat and cylindrical.
2. Mix the salt, pepper and dried herbs together. Lightly coat the fillet in olive oil, then rub it all over with the herb mix. Set aside at room temperature for 30-60 minutes.
3. For the horseradish butter, beat the butter in a bowl until soft, then mix in the garlic, horseradish, chives and a pinch of salt and pepper. Spoon it into a bowl and set aside somewhere cool, but not in the fridge, until needed.
4. Heat the barbecue to a high heat: 30-40 minutes in advance for charcoal, 10 minutes for gas.
5. Meanwhile, prepare the chunky potato gratin. Butter a 2 litre baking dish and heat the oven to 180°C/160°C fan/gas 4. Cut the potatoes into 6mm slices. Bring the milk and cream to the boil in a large saucepan and season to taste with salt and pepper before it gets too hot. Stir in the thyme leaves and sliced potatoes, then simmer gently for around 10 minutes, carefully turning now and then, until just tender when pierced with the point of a knife. Tip into the dish, shake to level, then sprinkle the cheese over the top. Slide the potato gratin into the oven and bake for 30-35 minutes until richly golden and bubbling hot.
6. Put the beef fillet on the barbecue directly over a high heat and sear for 15 minutes, turning every 3-4 minutes, or until browned all over. Re-arrange the coals/turn off the middle gas burner and lower the gas heat to medium.
7. Put the fillet over the indirect heat, cover with the barbecue lid, then cook, turning once halfway through, until it reaches your desired finish: 10-15 minutes for medium-rare (the internal temperature should be 52-55°C when tested with a thermometer); 25-30 minutes for medium to well done (60-65°C).
8. Put the beef on a carving board, cover with a loose tent of foil, then leave to rest for 5-10 minutes. Remove the gratin and leave that to rest as well.
9. Uncover the beef fillet, then snip and remove the string. Carve the meat into 1cm-thick slices, then serve with small spoons of the horseradish butter, the chunky potato gratin and salads.