Preheat broiler.
Slice rolls in half horizontally, leaving the 12 rolls attached.
Place both halves, cut side up, on a baking sheet and broil for 2 minutes, until golden. Be careful not to burn!
Remove the rolls from the oven and reduce heat to 400°
Meanwhile, brown ground beef along with onion until no pink remains and the onion is tender. Drain. If desired, add your preferred stir ins along with ⅓ cup of your preferred sauce. (But they taste great without stir ins and sauces, if that's your preference.)
Spread meat as evenly as possibly over the bottom half of the rolls. Top with slices of American cheese. Replace the top half of the rolls.
Bake for 10 minutes to warm through and melt the cheese. (If you prefer softer sliders, cover the rolls with foil and bake for 15 minutes instead.)