In a 12-inch cast-iron skillet, melt 3 tablespoons butter over medium-high heat. Add onion; cook, stirring frequently, until light golden brown, about 15 minutes. Reduce heat to mediumlow; cook, stirring frequently, until deep golden brown, about 20 minutes more. Remove from pan, and set aside.
In a medium bowl, stir together beef and steak seasoning. Shape mixture into 4 patties; place in skillet. Using a spatula, smash patties into flat disks. Cook beef, turning once, until well done, about 8 minutes. Remove from skillet, and wipe skillet clean.
Spread mayonnaise onto each bread slice. Top half of bread slices with 1 slice cheese, 1 patty, 2 tablespoons caramelized onions, and another 1 slice cheese. Cover with remaining bread slices, mayonnaise side down.
In same skillet, melt 2 tablespoons butter over medium heat. Add 2 sandwiches, and cook until golden brown, 2 to 3 minutes. Turn, and cook until cheese is melted and both sides are golden brown, 2 to 3 minutes more. Repeat procedure with remaining butter and sandwiches.
Notes
*We used Hellmann’s.
Recipe by Southern Cast Iron at https://southerncastiron.com/classic-patty-melts/