Add the oil to a griddle or a large cast iron skillet over medium heat, about 350°F.
Next, add the onions, gently season with salt, and cook, stirring occasionally, for 20 minutes, or until well browned.
In the meantime, spread the freshly ground beef into little mounds on a sheet tray lined with parchment paper or on a platter.
Once the onions are done, set them aside.
On the griddle, add half of the meat and spread it out across the griddle. Then, using two spatulas, press down on the beef to flatten it as best you can.
Immediately, evenly sprinkle on the adobo and sazon seasoning. You can substitute with 2 teaspoons of salt and a half a teaspoon of ground pepper.
Let the beef stand for 2 to 3 minutes. Next, using a spatula or a griddle scraper, chop the beef into smaller pieces.
Mix in half of the caramelized onions and then spread on half of the American cheese slices.
Once it begins to melt, chop in the cheese until it is completely mixed in. Once combined, set the cheesy beef to the side on a plate. Repeat the process with the remaining half of the beef. If you have a black stone or an extra-large griddle, you can cook the beef all at once.
Brush 2 teaspoons of mayonnaise on each half of the sliced rolls. Place them cut-side down on the griddle and cook for 2 to 3 minutes, or until toasted.
Remove the toasted rolls and evenly spread on the remaining mayonnaise and ketchup on the toasted buns.
Next, evenly add the cheesy beef to the bottom toasted bun, then layer on sliced tomatoes and shredded lettuce.
Fold the roll over, wrap in foil, and let stand for 1 to 2 minutes. Slice and serve.
Notes
Remember: we’re not going for taco meat here. You want to use a wooden spoon, two metal spatulas, or a griddle scraper to chop up the beef into a mix of small, bite-sized pieces and large, juicy chunks. These mismatched sizes are what set a chopped cheese apart from a cheesesteak and other ground beef sandwiches.Brown the onions for the full 20 minutes. Trust me: a little time and patience make all the difference in the sandwich's overall flavors.Don't just lay the cheese on top and let it melt. We want to get a little aggressive with it! Chopping the cheese right into the beef is how you get it to bind with the beef fat, onion juices, and crispy seared bits.Wrap the sandwich before slicing: Wrap the whole thing tightly in aluminum foil, let it sit for 1 to 2 minutes, and then cut it in half. This traps steam inside just long enough to soften the roll and glue the cheese, meat, lettuce, and tomato together so your sandwich doesn’t fall apart when you slice into it. Ground beef substitutes: You can also make a chopped cheese with ground turkey or chicken. I recommend adding an extra drizzle of oil to the griddle to help the edges crisp up without drying out the meat.Optional add-ins: For even more flavor, try caramelizing mushrooms with the onions, adding a dash of Worcestershire sauce, or chopping crispy bacon into the beef mixture. As for the toppings, add some pickled jalapeños, pickled cherry peppers, sliced banana peppers, or a splash of hot sauce. Make-Ahead: Definitely try to eat this sandwich right away so that it doesn’t get too soggy. For a head start, caramelize the onions, brown the beef, and chop the cheese 2 to 3 hours before serving. Just keep everything in a covered skillet over low heat until it’s time to assemble.How to Store: Place the chopped cheeseburger mixture in an airtight container, separate from the rolls, lettuce, and tomatoes. Refrigerate for 4 to 5 days. You can also freeze the cooked meat and onion mixture for up to 2 months. Let it thaw in the fridge before reheating.How to Reheat: Warm the leftovers in a covered skillet over medium-low heat with a tiny splash of water or oil. Or microwave the cheesy meat in 30-second intervals until heated through. Serve on a freshly toasted roll with toppings.