01 -
Preheat the oven to 350°F.
02 -
In a large frying pan over medium heat, melt 3 tablespoons of butter. Add the sliced onions and cook on low heat for 10-12 minutes, stirring often, until they begin to brown. Add minced garlic and rosemary, and continue cooking for an additional 5 minutes until golden brown. Remove from heat and set aside.
03 -
Line a sheet pan or baking tray with parchment paper. Place the slider buns on top and slice them through the center, setting the tops of the buns to the side.
04 -
Layer the bottoms of the slider buns with the roast beef. Add slices of cheese on top of the roast beef, using 4-6 slices or enough to cover. Distribute caramelized onions evenly over the cheese, then place the tops of the buns back on.
05 -
In a small bowl, mix melted butter, mustard powder, garlic powder, and parsley until evenly combined.
06 -
Use a brush to spread the garlic butter evenly over the tops and sides of the slider buns.
07 -
Place the sliders in the preheated oven and bake for 5-8 minutes, or until the cheese has melted and the buns are toasted.
08 -
Remove from the oven and allow to cool for 2-3 minutes. Pull the sliders apart or cut them with a knife. Serve and enjoy.