Start making the beef jus. Heat a medium saucepan with olive oil and add the diced onion cooking over a medium heat. Cook until the onion is soft and starting to brown.
Add the beef broth, Worcestershire sauce, peppercorns and dry sherry and reduce down on a gentle heat until 1/3 reduced.
Strain the liquid through a sieve and discard the pepper corns and onion.
Put the liquid back in a pan, check for seasoning with salt and pepper and keep warm.
In a separate pan add 50 grams of butter and 3 brown onions halved and thinly sliced. Gently cook the onions for about 45 mins until nicely caramelised.
Slice the rolls lengthways and spread remaining butter.
Place the rolls under a grill butter side up to lightly toast.
Remove from the grill and spread the toasted sides with horseradish cream and grain mustard.
Take the roast beef and divide equally between the sandwiches. Top with a slice of jarlsberg cheese and place back under the grill to gently melt.
Once the cheese has just started to melt remove from the grill and top with a tablespoon of the caramelised onions.
Close the sandwich, slice in two and serve each sandwich with a small bowl of beef jus to dip the sandwich into between bites.