In a small mixing bowl, combine the mayonnaise, prepared horseradish, Dijon mustard, freshly squeezed lemon juice, and a pinch of salt and pepper. Mix until smooth and well-combined.
Using your sharp knife, thinly slice the medium red onion and the medium tomato. Set aside the slices for later assembly.
If you prefer a little crunch, toast your whole grain rolls in a toaster or on a skillet until they are lightly golden.
Spread a generous amount of the horseradish mayo on both sides of the toasted rolls. Start layering by placing 4 slices of roast beef on the bottom half of each roll. Follow with 2 slices of Swiss cheese, a handful of baby arugula, a few slices of red onion, and tomato. Season with salt and pepper to taste.
Top with the other half of the roll, pressing down gently to combine all the flavorful ingredients. Cut the sandwich in half if desired, and serve immediately.
