Place the oven rack in the middle position and preheat the oven to 350 degrees.
Slice the bread rolls in half, horizontally. Spread 4 tablespoons of the yellow mustard evenly on the cut sides of the roll tops and bottoms. Place the roll bottoms cut side up in a 13 inch by 9 inch baking dish.
Fold the ham slice into thirds, and then in half. Place one piece of ham on each bottom. Fold the Swiss cheese in the same manner as the ham and place on top of the ham. Season with freshly ground black pepper, to taste. Top with the roll tops.
In a medium bowl, combine butter, onion, and poppy seeds. Place in the microwave and heat until the butter is melted and the onions are softened about one minute. Whisk in the Worcestershire sauce, the garlic powder, and the two remaining tablespoons of mustard.
Brush the sides and the tops of the sandwiches liberally with the poppy seed mixture using a pastry brush, and spoon the remaining solids over the tops. Cover the dish with aluminum foil and allow time for the sandwiches to absorb the sauce, about ten minutes.
Bake for 20 minutes. Remove the foil and bake uncovered until the cheese has melted and the tops are slightly firm, about 7 to 9 minutes. Allow the sandwiches to cool, about 10 minutes. Serve.