Roast beef sirloin with spiced celeriac gratin

Method

1. Heat the oven to 180ºC/160ºC fan/gas 4. For the gratin, put the cream, milk, garlic, nutmeg and thyme in a large pan. Season, bring to the boil (don’t worry if it curdles), then take off the heat and leave to infuse.
2. Meanwhile, peel and quarter the celeriac, then cut each quarter across into slices the thickness of a 10p coin (use a mandoline if you have one). Drop the slices into the hot milk and cream mixture as you do each quarter, turning them once or twice to coat and stop them browning.
3. Lightly butter the dish, then spoon in the celeriac mixture, pressing so the layers are level. Cover with buttered foil, and bake for 1 hour.
4. Remove the gratin from the oven and set aside. Increase the heat to 230ºC/210ºC fan/gas 8. Heat the oil in a large heavy-based frying pan over a medium-high heat. Add the beef and sear on all sides for 10-12 minutes until nicely browned (3 minutes for the top and bottom of the joint and briefly at each end).
5. Put the beef in a large roasting tin and season with salt and pepper. Roast for 20 minutes for rare, 25 for medium-rare or 30 for medium. Remove from the oven, put on a board and cover tightly with foil. Leave to rest for 10-15 minutes.
6. Meanwhile, uncover the grain and sprinkle over the grated cheeses. Return to the oven and bake, uncovered, for 10-15 minutes until bubbling and golden.
7. Carve the beef across the grain into thin slices, then serve with the sauce and seasonal veg, if you like.