Preheat a cast-iron skillet over medium-high heat for 5 minutes.

Press the ground beef into a thin, even layer and season with adobo, sazón, and black pepper. Let sear undisturbed for 2–3 minutes to develop a nice crust.

Flip the beef, add the diced onion, and use two metal spatulas to chop everything together. Cook for another 3–4 minutes, until the beef is fully cooked and finely chopped.

Divide the beef into two roll-length portions. Top each with 2–3 slices of American cheese, cover, and let melt for about 30 seconds. Chop again to mix that melty cheese throughout the beef.

Push the meat aside, spread mayo on the rolls, and toast cut-side down in the skillet until golden.

Load each toasted roll with the cheesy beef mixture, then pile on shredded iceberg lettuce, sliced tomato, and ketchup.

Wrap each sandwich tightly in foil for 1–2 minutes to steam and bring everything together. Slice in half and enjoy HOT!