Heat a large griddle over medium-high heat and form the ground beef into four loose oblong portions.
Set the portions on the griddle, scatter the onion around them and season everything with salt, black pepper and garlic powder.
Cook for 3 minutes, flip the beef and chop it into the onions until no pink remains and the meat reaches 160°F.
Divide the filling into four piles, place two American cheese slices over each and fold the melting cheese through the beef.
Spread 1 tablespoon mayonnaise inside each roll and divide the hot filling among them.
Add 1/2 cup lettuce and several tomato slices to each roll, then close and wrap for 1 minute before serving.