Place steaks in the freezer for 30-40 minutes before slicing. This will make shaving steaks thinly very easy. Cut partially frozen ribeyes in half widthwise and shave them into 1/8 inch thick strips. Keep some fat and discard some fat as desired.
Preheat the oven to 350 °F.
Heat 1 tablespoon of oil in a cast-iron skillet on medium heat. Add thinly sliced onions and saute until golden and tender. Season with salt and pepper and transfer to a small bowl.
Add a touch more oil to the pan and add shaved ribeye steak. Season liberally with salt and pepper. Saute until fully cooked through and no longer pink, about 5-7 minutes. Use the tip of a metal spatula to break up long strips into small bite-sized chunks. Return the onions to the skillet and mix with the beef.
Arrange the bottom buns of the sliders on a greased 11×17 pan (no need to break apart attached buns). Spread an even layer of beef/onions over the buns. Top with a layer of American cheese slices and then a layer of provolone slices. You want a complete sheet of cheese coverage with no overlap.
Bake sliders without the top buns for 5 minutes or until cheese is nice and melted. Remove from the oven, place top slider buns on cheesesteaks, and lightly brush with melted butter. Return to the oven for about 5 more minutes.
Cut slider borders with a knife if needed and serve immediately.