Let the beef roasting joint come to room temperature 1 hr before roasting. Heat the oven to 220C/200C fan/gas 7. Pat the meat dry using kitchen paper, then combine the oil, mustard, onion salt, all-purpose seasoning and a good grind of black pepper. Rub the mixture all over the beef joint. Arrange the onions in a single layer in the base of a large roasting tin, and pour in a generous cup of water. Sit the beef on top (fat-side up, if there is any).

Roast for 20 mins, then reduce the oven to 180C/160C fan/gas 4 and roast for another 55 mins for medium (it should read around 48C when a meat thermometer is inserted into the thickest part of the joint). Remove from the oven and leave to rest in the tin, covered, for 20 mins.

Remove the beef to a chopping board and slice as thinly as you can. Set aside on a plate. Strain the roasting tin juices into a pan, reserving the onions on the plate with the beef. Bring the strained liquid to the boil, then reduce the heat and simmer for 3 mins until reduced. Stir in the butter and continue to simmer for 2-3 mins until thickened. Remove from the heat.

To assemble the sandwiches, heat the oven to 180C/160C fan/gas 4. Split the baguettes and spread the reduced resting juices over the bases. Top with the horseradish, roasted onions, beef and smoked cheese, then bake on a tray in the oven for 5 mins to warm through and melt the cheese slightly. Scatter in a handful of rocket and some crispy onions before serving.