Directions

1. Heat the butter or olive oil in a large skillet over medium-low heat.
   
2. Add the thinly sliced onions to the skillet and spread them out evenly.
   
3. Cook the onions slowly, stirring occasionally, until they become golden brown and caramelized, about 25-30 minutes. 
   
4. If adding sugar or balsamic vinegar, sprinkle it over the onions about halfway through the cooking process.
   
5. Once the onions are caramelized to your liking, season with salt to taste.
   
6. For the burger, in a different skillet, season both sides of the ground beef patty, then cook on both sides until the beef is cooked all the way through at a temperature of 145°F.
   
7. Mix Japanese mayo, horseradish, black pepper, and vinegar in a bowl.
   
8. To assemble the burger, spread sauce on the bottom bun, add the cooked patty, top with caramelized onions, and then place the Swiss Cheese slices on top. Torch the cheese with a kitchen-grade torch, top with bun and serve.