Mix together the mayo, mustard, poppy seeds, grated onion, horseradish mayo and Worcestershire. Taste the dressing and adjust the ingredients however you wish.
To assemble, spread the dressing on the top and bottom buns. On the bottom bun, lay on some provolone, roast beef and American cheese, and place the top bun on top. Repeat with the next sandwich.
Wrap each sandwich in a foil square, and either bake right away or keep in the fridge. To bake right away, put them on a baking sheet in the oven until the buns are slightly crusty and the cheese is melted, 15 to 20 minutes. Great for a crowd!