Whisk in the onion, mustard, poppy seeds, and Worcestershire sauce.
Brush the butter mixture evenly over both the top (outside) and bottom half of the rolls.
Layer the ham and Swiss slices over the bottom half of the rolls.
Replace the top half of the rolls, and cover with foil.
Bake for about 15-20 minutes, or until cheese is melted.
Remove from oven, and serve warm.
Dinner rolls: I like to use Hawaiian sweet rolls for these sandwiches. You can make your own Hawaiian sweet rolls with this recipe. You can also use regular dinner rolls if you prefer.
Butter: I use salted butter. If you're using unsalted butter, add a pinch or two of salt to your butter mixture.
Mustard: You can use whatever type of mustard you like. I've made these sandwiches with everything from regular yellow mustard to Dijon mustard to horseradish mustard.
Ham: You can use leftover ham or thinner sliced lunchmeat.
Cheese: You can use another type of cheese if you prefer. I've made these sandwiches with white cheddar and sharp cheddar. If you use another type of cheese, be sure that it's thinly sliced so that it will melt well.
Melting the butter: You can also melt the butter in a microwave-safe bowl. You will want it just melted, not hot and sizzling.