Preheat oven to 325°. Mix together butter, mustard, Worcestershire, poppy seeds, and onion until smooth. Evenly spread cut sides of rolls with butter.
Stack ham, then cheese, inside rolls.
Set sandwiches on a baking sheet and cover with foil. Bake until warmed through and cheese is melted, about 20 minutes.
Note: Nutritional analysis is per sandwich.