Melt butter in a small skillet. Once melted add onions.
When onions begin to soften add mustard, Worcestershire sauce, and poppy seeds into onions and butter. Simmer for two to three minutes.
While the onions are cooking, layout buns. Place ham and cheese on one side of the half bun.
Once the onion-poppy seed mixture is done, add it to the other half of the bun.
To freeze, you have several options. I now just put each sandwich in an individual freezer bag. My parents would wrap the sandwiches in saran wrap and put the sandwiches into the bun bags to freeze. You do what works for you.
To cook, microwave for 1 minute to a minute 30 seconds on medium high heat. You can also bake for 15-20 minutes in the oven on 425 heat. I would recommend thawing the sandwiches for the oven and transfer the sandwich to aluminum foil wrap.