Adjust oven rack to middle position and heat oven to 350 degrees. Slice rolls in half horizontally. Spread 4 tablespoons mustard evenly on cut sides of roll tops and bottoms. Arrange roll bottoms, cut side up and side by side, in a 13 x 9-inch baking dish. Fold ham slices in thirds, then once in half; place 1 slice on each roll bottom. Fold Swiss cheese like the ham, then place over ham. Season with pepper and cover with roll tops.

Combine butter, onions and poppy seeds in bowl. Microwave until butter is melted and onion is softened, about 1 minute. Whisk Worcestershire sauce, garlic powder, and remaining 2 tablespoons mustard into butter mixture until combined. Generously brush tops and edges of sandwiches with all of butter mixture. Spoon any remaining solids over sandwiches.

Cover dish with aluminum foil and let sit for 10 minutes to allow sandwiches to absorb sauce. Bake for 20 minutes. Then, uncover and continue baking until cheese is melted around the edges and the tops are slightly firm, 7 to 9 minutes. Let cool for 10 minutes before serving.