Preheat oven to 350°F if baking immediately, or prepare to bake from chilled the next day at 350°F for 20 minutes. Line a 9x13 pan with foil for easy cleanup.
Slice rolls horizontally without separating; place bottom half in the pan. Layer ham and cheese evenly, then press top half of rolls on.
Whisk mayo, Dijon, honey, melted butter, onion powder, salt, and pepper. Pour or brush sauce over the tops, getting into seams.
Cover tightly with foil and refrigerate overnight. Next day, bake covered 15 minutes, uncover and bake 5 more until cheese melts and tops brown slightly. Let rest 5 minutes, then pull apart.