Mix together the mayonnaise, grated onion, poppy seeds, spicy mustard, Worcestershire, and horseradish in a medium bowl.
Spread some of the sauce on both sides of the bun.
Add a few slices of either beef or ham.
Place the wrapped sandwiches into two large resealable freezer bags. Freeze.
Remove the sandwiches from the bags, but keep them wrapped in the foil.
Cook them in a 350° oven right in the foil for 20-30 minutes (20 minutes if the sandwich is thawed and 30 minutes if cooking from frozen).