Preheat the oven to 350 degrees. Melt the butter and whisk together with mustard, Worcestershire, onion, and poppy seeds. Split the rolls –– if you are using the 24 pack, it can be easiest to treat it as one large roll. Spread the butter mixture on both sides of the roll.
I typically wrap them in foil before putting them in the oven to keep drips to a minimum. Bake for 20-25 minutes or until the swiss has melted and the tops are golden brown. With the King's Hawaiian rolls, you should get a nice toast on the top.
Serve warm (or eat cold from the fridge as leftovers the next day). Enjoy!