8 slices thick hearty bread rye is traditional 1 pound ground beef 1 white onion 3 Tablespoons butter 8 slices Swiss Cheese ¾ cup mayonnaise 1 teaspoon beef seasoning
Mix the beef and beef seasoning together. Divide the ground beef mixture into 4 even balls.
Spread the mayonnaise to one side of each piece of the sourdough bread with 1 Tablespoon of the mayonnaise.
Melt the butter on one side of the griddle. Then top the melted butter with the sliced onions. Sauté the onions in the butter until they are caramelized.
Place the ground beef on the grill. Put a piece of parchment paper on top of the beef and use a spatula or a burger press to smash the meat into thin patties.
Let the patties cook until they are browned and crispy (2-3 minutes). Then flip the patties over (do not smash) and cook the other side for 2-3 minutes until it is also browned and crispy.
Place 2 pieces of the Swiss cheese on each patty.
Put the bread on the Blackstone grill mayonnaise side down and toast the bread.
Place one piece of the bread on a plate, top with 1 of the patties, ¼ of the fried onions, 1 Tablespoons of mayonnaise and top with the 2nd piece of bread.