Preheat oven to 400 degrees F.
In a large mixing bowl, beat together 14 tablespoons of softened butter, poppy seeds, Worcestershire sauce, and mustard until blended.
Add the minced onion, ham and cheese and fold together with a wooden spoon until well mixed.
Slice the rolls in half lengthwise and place cut side up in a 13x9-inch baking pan lined with parchment or place on sheets of foil large enough to cover the rolls. Divide the ham mixture equally on top of the three pans of rolls. Cover with the top half of the rolls. Cover with a double thickness of foil to seal. (If desired, seal and refrigerate or freeze at this point.)
TIPS: If made ahead and refrigerated, bring to room temperature, about 30 minutes before baking. If frozen, thaw overnight in the refrigerator then set out for 30 minutes before baking.
Bake in a preheated 400 degree oven for 20 minutes or until heated through. If making 24 thicker rolls, bake for an additional 5 minutes. Melt the remaining two tablespoons of butter and brush on top of the hot rolls, if desired. Sprinkle with poppy seeds and serve.