Slice the onions, keeping them thin. In a medium pan, add the butter and the onions and let them get golden and caramelized. Stir to keep them evenly coated in the butter. Mix the ground beef with the soup packet and form burger patties. Grill them until they're done to your liking! While they're grilling up, make the au jus. Add the packet with 2 cups of water to a medium pot and simmer.
Top those hot burgers with a generous amount of swiss or gruyere cheese, cover them with a foil tent and let it get melty! Brush the buns with some warm au jus. Now stack that big juicy, cheesy burger. You'll need two hands and/or a knife and napkins for sure. Take that big juicy bite!