Season potato slices with salt and pepper to taste. Add 1 teaspoon olive oil to a skillet and heat over medium heat. Add sliced potatoes and cook on both sides until lightly browned.
Remove potatoes from the skillet and set aside. Add onion to skillet and drizzle with balsamic glaze. Cook onions until soft. Remove onions and set aside.
Spread mustard on one side of the sandwich bun and horseradish mayo on the other side. Repeat for second sandwich.
Add roast beef, potatoes, onion, and cheese. Top with bun.