Assemble the sandwiches: Slice the rolls in half horizontally and place the bottom halves in the baking dish. Layer 1-2 slices of ham and cheese on each roll and cover with roll tops.
Make the sauce: In a small bowl mix together the melted butter, mustard, grated onion, and Worcestershire sauce. Spoon the sauce over the tops of the sandwiches. Allow rolls to rest for 10-15 minutes before baking, or refrigerate for up to 1-2 days (depending on how fresh your ingredients are).
Bake at 350 degrees F for about 15 minutes, or until warmed through and the cheese is melted.
Make Ahead Instructions: Assemble sliders completely, cover and store in the fridge for 1-2 days (depending on how fresh your ingredients are). Remove from fridge 15 minutes before baking, and bake as instructed.Freezing Instructions: Assemble sliders as instructed (in a freezer-safe dish or aluminum pan), including the sauce on top. Cover with a double layer of aluminum foil, sprayed with cooking spray so it doesn’t stick to the rolls. Freeze for up to 3 months. Bake from frozen until the roll tops are golden and the cheese is melted, about 18-25 minutes