Patty melts are one of my favorite diner foods, and as it turns out, they're pretty darn good made at home too (although they take quite a bit longer on account of the caramelized onions!). I'd suggest caramelizing the onions in a big batch and freezing the extra, because they're a pain to make but so delicious. You can definitely make them ahead and then the rest of the recipe comes together easily. In order to imitate the heat of a diner flat-top, the burgers are broiled, first on their own to cook the meat, and then with the onions and Swiss cheese to get the cheese all bubbly. Then the burgers are sandwiched between slices of rye bread, buttered, and cooked similar to a grilled cheese sandwich. The result is an ultra-decadent sandwich - so good!
Best-Ever Patty Melt
2 medium or 1 large yellow onion, thinly sliced
Salt and pepper
1 to 1-1/2 pounds ground beef (depending on how large you'd like your burgers)
Thinly sliced Swiss cheese
8 slices rye bread
Ketchup, mayo, and/or other burger fixings
1. Heat a non-stick skillet over medium heat. Melt a tablespoon or two of butter, and then add the onions. Slowly caramelize the onions until they are deeply browned. Add more butter as needed, and turn the heat down and stir more often as the process progresses -- it should take at least 40-45 minutes, so this is a good task to do while you're working on something else in the kitchen. Season lightly with salt, and then use immediately or store for when you're ready to make the burgers.
2. Preheat the broiler to high. Line a baking sheet (or your broiler pan) with foil.
3. Season the beef with salt and pepper, and divide into four portions. Gently press each portion into a thin, oval-shaped patty (basically, as thin as you can get it while still holding together). Arrange on the baking sheet.
4. Broil the burgers for 3 minutes, then flip. Divide the onions among the burgers, and top with Swiss cheese. Broil for another minute or so, until the cheese is bubble.
5. Butter each slice of bread on one side, and sandwich the burgers between the bread so the buttered sides are on the outside.
6. Heat a non-stick skillet over medium-high heat. Add the burgers (working in batches if necessary), and cook as you would a grilled cheese sandwich (i.e. on both sides until browned and crispy). Top with whatever burger fixings you prefer, and serve hot.