Prepare the sauce by mixing mayonnaise, Dijon mustard, horseradish, lemon juice, and black pepper in a small bowl.

Thinly slice the red onion and tomato; wash arugula or greens.

Toast the sliced bread lightly in a skillet with butter or in a toaster.

Warm the sliced roast beef in a skillet over low heat with a splash of beef broth or water.

Assemble the sandwich by spreading the sauce on toasted bread and layering with beef, cheese, greens, onion, and tomato.

Cut the assembled sandwich in half and serve immediately.