1/3 to 1/2 c Cheez Whiz or 1/2 of your favorite cheese, grated
Peperoncini, for serving
Slice the roast beef into thin, julienne strips. Set aside.
Heat the olive oil in a large skillet over medium-low heat. Add the onion and cook, stirring occasionally, until very soft and golden, about 20 minutes. Add the roast beef and cook, stirring to heat through, about 1-2 minutes. Season with salt and pepper.
Place 2 tortillas on a work surface and scatter evenly with the onions and beef. Use a spoon and a small spatula to dollop the Cheez Whiz or your favorite cheese evenly over the onion and beef mixture. Place a second tortilla over each, and press down with the palm of your hand.
Heat a skillet over medium heat until it is very hot. Use a large spatula to slip a quesadilla into the skillet and cook, pressing down with the spatula and turning once, until the tortilla browns slightly, 3 to 4 minutes per side.
Cut the quesadilla into quarters and serve hot with peperoncini on the side.