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.
Top with a slice of cheese.
Wrap each sandwich in tin foil.
Place the wrapped sandwiches into two large resealable freezer bags. Freeze.
Cooking instructions:
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) or cook in the foil on the BBQ until the cheese is melted.