Slice bread loaf in half horizontally. Dig out excess bread, leaving approximately 1-inch on the bottom and sides.
Spread both halves of the bread generously with Dijon mustard, according to your taste.
Start building the sandwich with roast beef, add salt (optional) and pepper to taste, Provolone cheese, red onion, dill pickles, and arugula leaves.
Replace the top and slice into individual servings.
Gourmet Roast Beef Sandwich Recipe: https://whatscookingamerica.net/sandwich/rstbeefsand.htm