When ready to cook, set Traeger temperature to 500°F and preheat, lid closed for 15 minutes.
Season the roast liberally with Traeger Beef Rub and salt.
Place the roast directly on the grill grate and cook for 30 minutes until the exterior starts to caramelize and brown.
Reduce the grill temperature to 325℉ and continue to cook roast until the internal temperature reaches 125℉, about 30 minutes.
Remove roast from grill and let rest for 15 minutes before slicing.
After resting, thinly slice the meat on a meat slicer or with a very sharp knife. Set meat aside.
Place butter and onions in a sauté pan and cook over medium heat. Season liberally with salt and cook, stirring frequently, until the onions are browned and caramelized. If the bottom of the pan starts to stick before the onions are completely cooked through, add 1 tablespoon of water, scrape up the browned bits with a wooden spoon and continue to cook. When completely softened and caramelized, remove from the heat and set aside until ready to build the sandwiches.
Place the ingredients for the au jus in a pot and bring to a simmer. Cook for 30 to 45 minutes, season liberally with salt and strain the solids out.
Toast the buns, cut-side down, on the grill for 5 to 10 minutes until lightly browned.
Place them toasted side up on a sheet tray. Spread the mayonnaise on the bun, top with the thinly sliced meat, then caramelized onions and cheese.
Place the tray directly on the grill grate with the temperature set to 500°F and cook just until the cheese is melted.
Serve sandwiches hot with Au jus. Enjoy!