2In a skillet heated to medium, melt the 2 tablespoons butter. Add the onions and cook on low for about 10 minutes until golden brown and caramelized. Remove from heat.
3Cut the entire 24-roll package in half so the bun tops are on one side and the bottoms on the other.
4Onto the bottom half, layer the roast beef, then cheese, then spoon caramelized onions evenly over the top.
5If making ahead, stop here and place in fridge until ready to bake. Remove from fridge when you preheat the oven.
6In a microwave-safe bowl, melt the stick of butter in 30-second intervals until completely melted. Add rum, brown sugar, Worcestershire sauce, and mustard; stir well until the sugar is completely dissolved.
7Spread about half of the butter mixture on the inside of the bun tops. Place bun tops over the bottoms in a baking dish. Brush the remaining butter mixture over the top and sides of all the sliders.
8Bake at 350°F until the cheese is melted and the tops are golden, about 15–20 minutes.
Notes
Great make-ahead party food for game day or Super Bowl. Can be assembled through step 4 and refrigerated; add butter glaze just before baking.