Preheat oven to 350°F. Line a 9×13-inch pan (or smaller if it fits the rolls) with parchment paper. Alternatively, use aluminum foil sprayed with cooking spray.
Cut: Using a large serrated knife, slice the rolls in half to separate the top from the bottom. Do not separate individual rolls.
Spread mustard: Place the bottom half of the rolls and cover with 1 tablespoon of Dijon mustard.
Add ham and cheese: Cover with ham and cheese. You can alternate layers, if preferred, as long as it is evenly spread across all the sliders. Set top of rolls on top.
Heat butter: In a medium microwave-safe bowl, add the butter and melt in microwave. For best results, heat in one minute increments (or less) using 50% power.
Mix: Add the remaining Dijon mustard, poppy seeds, garlic powder, crushed fried onion pieces, and Worcestershire sauce. Stir to combine.
Add topping: Spoon the butter mixture over the rolls to cover evenly. Use all of it.
Bake in oven:
Cover and heat: Cover tightly with aluminum foil and bake in preheated oven for about 30 minutes until cheese has melted. This might take even longer depending on starting temperature of ingredients, size of pan, etc. Cheese around edges will melt much faster than the cheese in the center.
Toast: Uncover and cook for just a few minutes to slightly toast the tops.
Slice and serve: Remove from pan by lifting parchment paper, slice into individual sliders using a large sharp knife, and serve immediately.
Nutritional information based on one slider per serving.