Slice the white onion in slices along the direction of the rings
In a small pan, heat up some olive oil and brown the onions. When browned to your liking add a litte butter and cook until onions are soft.
Cut the buns and butter each side generously.
Put a cast iron skillet on medium high heat. While skillet is warming up, toast the buns until browned to your liking.
Test the hotness of the skillet, by adding a small droplet of water. The water should immediately evaporate.
Add the beef balls to the skillet, evenly spaced. Put the wax paper on top and with the heavy weight pressing down on the spatula smash the beef balls as thin as you can.
When the edges begin to brown, flip over and add cheese. Close skillet with lid so steam melts the cheese.
When cheese is melted, stack smashed patties for a double.
For the burger sauce, mix mayonaise, garlic sauce and perinaise. Mix well.
To assemble, add sauce to bottom half of bun, add smashed patties, then onions. Add more sauce to top half of bun and close.