Melt butter and mix in mustard, Worcestershire, minced onion and poppy seeds.
Slice rolls in half horizontally. Place bottom halves in a baking dish and cover with ham and cheese.
Cover ham and cheese with the top halves of the rolls.
Drizzle butter mixture over the top of rolls, distributing evenly.
Refrigerate at least 1 hour to allow sauce to soak in.
Bake uncovered at 350 F for 15-20 minutes, until heated all the way through and cheese is melted. Separate individual sandwiches for serving.