Keeping all 9 slider buns together, place tops (topside up) and bottoms (open faced) on sheet pan. Brush melted butter on top of slider buns. Sprinkle sesame seeds on the top buns. Cover loosely with foil and toast in oven for about 5 minutes. When done remove from oven and separate slider buns.
While buns are toasting, cook slider patties to your preferred temperature, seasoning with salt & pepper. Add cheese to the top for maximum melting action.
Quickly build your sliders while slider patties are hot: Bottom slider bun, one slice of cheese, slider patty with cheese, pickles, ketchup, mustard, chopped onion, top slider bun.