Slice the roast beef into thin, even pieces.
In a bowl, mix mayonnaise and Dijon mustard until combined.
Slice the sandwich rolls in half and lay them out.
Spread the mayonnaise mixture on both sides of each roll.
Layer the sliced roast beef on the bottom half of each roll.
Add sautéed onions on top of the roast beef.
Place a slice of cheese on top if desired.
Add pickles on top of the cheese or beef.
Season with salt and pepper to taste.
Close the sandwich with the top half of the roll and serve.