Preheat the oven to 350° fahrenheit. Combine mayo, mustard, poppy seeds, grated onion, horseradish mayo, and Worcestershire sauce In a bowl. You can adjust the ingredients according to your taste.
Assemble the buns by spreading the dressing on the top and bottom buns.
Place provolone, roast beef, and American cheese on the bottom bun, and top with the top bun. Continue with the next sandwich.
Wrap each sandwich in foil and bake as soon as possible or store in the refrigerator.
For right away baking, place the sandwich on a baking sheet and bake in the oven for 15 to 20 minutes, until the buns are slightly crusty and the cheese has melted. Enjoy your roast beef sandwich!