Heat oven to 350°F. Lightly grease a 9x13-inch baking dish.
Slice the rolls horizontally as a single slab. Set the bottoms in the baking dish.
Spread the cut sides with mayonnaise, if using.
Layer ham evenly over the bottom slab. Top with the Swiss cheese slices.
Place the top slab of rolls over the cheese to close the sandwiches.
Whisk melted butter with Dijon, Worcestershire, poppy seeds, dried onion, garlic powder, brown sugar, salt, and pepper.
Brush the butter mixture over the tops and down the sides. Spoon any extra into the edges.
Cover the dish tightly with foil and bake for 12 minutes.
Uncover and bake 3–5 minutes more, until the tops are lightly toasted and the cheese is melted.
Rest 5 minutes. Slice along the roll lines and serve warm.