Combine horseradish and mustard and stir well. Slowly wisk in the melted butter until it is fully incorporated and emulsified. Add poppy seeds.
Open hamburger buns and brush both sides of the inside with the poppy seed dressing. Place two ounces of ham and one slice of cheese on bottom part of bun. Repeat with the remainder of the buns. Close the tops of the buns and brush more of the poppy seed dressing on the outside tops and bottoms of buns. Tightly wrap each sandwich in aluminum foil and freeze.
To cook, preheat oven to 400-degrees. Place sandwich, still tightly wrapped in foil, directly on the center rack for approximately 30-45 minutes until center is hot and cheese is melted.
Yield: eight sandwiches