Caramelize the onions by heating a skillet over medium heat, adding thinly sliced onions and a pinch of salt. Cook for about 10 minutes until golden brown.

Cook the ground beef in the center of the skillet for about 5-7 minutes, breaking it apart until browned. Drain excess fat.

Mix caramelized onions back into the skillet with the beef, adding soy sauce, salt, and pepper. Cook for another 3-5 minutes.

Toast the buns in a preheated oven at 350°F for about 5 minutes until lightly toasted.

Assemble the sandwiches by filling toasted buns with the beef mixture, then top with cheese.

Melt the cheese by covering the assembled sandwiches and returning them to the oven for an additional 3-5 minutes.

Serve warm, optionally garnished with extra caramelized onions or fresh herbs.