Preheat an oven to 160°c/140°c fan (320°f/284°f).
Thinly slice the roast beef, season with salt & pepper then place into a baking dish.
Next, warm the gravy up in a pan (or in a microwave) then once hot, pour it all over the beef. Cover the dish with foil then bake in the preheated oven for 15-20 minutes, until piping hot.
When your beef is almost ready, cut the burger buns in half then toast them, either in a frying pan set over a medium heat or under a grill. Once toasted, place the bottom halves onto a baking tray.
Top each bun (just the bottom half!) with a teaspoon of horseradish sauce then spread it out to completely cover the bread.
Next, divide the slices of beef between the sandwiches, placing it on top of the horseradish. I'd recommend using a pair of tongs to lift the beef out of the gravy then spoon extra gravy into the sandwiches! Top each sandwich with 2 slices of Swiss cheese then place under a grill for a minute or so, to melt the cheese. If you haven't got a grill, bake in the preheated oven for a couple of minutes, or use a blowtorch to melt the cheese.
Once the cheese is melted, top the sandwiches with the top half of each bun then serve.