For the Roast Beef: 

Sprinkle roast evenly with the salt and pepper and refrigerate on a wire rack set in a rimmed baking sheet uncovered for at least 1 hour or up to 24 hours.

Adjust oven rack to center position and preheat oven to 225°F (105°C). Place baking sheet with rack and beef in oven and roast until internal temperature registers 115°F (46°C) on an instant-read thermometer at the thickest point, about 1 to 1 1/2 hours.

Just before the roast comes out of the oven, add oil to a 12-inch stainless steel, or cast iron, or carbon steel skillet and heat over high heat until smoking. Add roast to skillet and cook, using tongs and moving roast as needed, until roast is evenly browned on all sides, and internal temperature registers 125°F (50°C), 5 to 7 minutes.

Transfer the roast to a large plate and let cool completely, about 1 hour (if time allows, refrigerate the roast whole until well chilled, at least 2 hours and up to 1 day). 

Slice the roast as thin as possible against the grain. 

For the Sandwiches: 

In a 12-inch stainless steel or cast iron skillet, melt 2 tablespoons butter over medium heat until just melted. Add 2 bun bottoms and 2 bun tops, cut side down, and cook, swirling bread around the skillet frequently, until toasted evenly on one bottom, about 3 minutes. Transfer buns, toasted side up, to a plate and while still hot, add a slice of cheese to each top and bottom. Repeat with remaining 1 tablespoon butter, remaining bun halves, and remaining cheese.Transfer to a plate.

Spread a thin layer of mayonnaise, if using, over bun bottoms. Divide the sliced roast beef and mound it evenly on the roll bottoms (about 6 ounces beef per sandwich). Spread barbecue sauce, if using, or your preferred sauce over the beef and add your favorite toppings. Close sandwiches with bun tops and serve.