In a large skillet, melt a tablespoon of butter over medium heat. Add thinly sliced onions and sauté for about 10-15 minutes, stirring occasionally, until they turn golden brown and caramelized.

Once the onions are caramelized, push them to the side of the skillet and add the lean ground beef to the center. Increase heat to medium-high and cook the beef for about 6-8 minutes until browned.

Mix the caramelized onions back into the skillet with the ground beef, then pour in the gluten-free soy sauce and sprinkle in seasonings. Stir well for 2-3 minutes.

Reduce heat to low and sprinkle cheese over the beef mixture. Cover with a lid and allow to steam for about 3-5 minutes until the cheese melts.

Toast the buns in the oven for about 5 minutes at 350°F. Spoon generous portions of the beef and cheese mixture onto the toasted buns.