A patty melt is a sandwich consisting of a hamburger patty, melted cheese (I use Cheddar), and caramelized onions, all placed in between two slices of bread – it’s essentially a hybrid burger and sandwich.
Heat a large sauté pan over medium heat, add the olive oil and heat through. Add the butter and allow to melt. Add the onions and toss quickly to remove the moisture from the onions. Sauté, stirring occasionally, until the onions are soft and start to brown.
Sprinkle the salt and sugar over the onions (to help them caramelize) and reduce the heat to medium-low and cook, stirring occasionally, until the onions are light brown and begin to caramelize, about 20 minutes. Add the balsamic and stir to combine.
Continue to cook until the onions are uniformly brown and caramelized, about an additional 10 minutes. Remove from the pan and set aside or cover and refrigerate until ready to use.
For the sriracha mayonnaise:
In a small bowl stir together the mayonnaise and sriracha until completely combined. Cover and refrigerate until ready to use.
For the melt:
Form the meatinto 4 equal square-shaped patties and season on both sides with salt and pepper. Heat a large cast iron pan over medium-high heat. Working 2 patties at a time, add to the pan and cook until a browned crust forms, about 3 minutes. Flip and continue to sear until cooked to medium, about an additional 2 minutes. Remove from the pan and set aside, covered on a plate. Repeat with the remaining patties.
Wipe out the pan and reduce heat to medium-low, add the vegetable oil and heat through then add 1 tablespoon of butter and allow to melt. Working in batches, add 2 slices of bread to the pan top with each with 2 slices of cheese, then follow with beef patties, a spoonful of caramelized onions and the remaining slices of Cheddar cheese. Spread the remaining 2 slices of bread with about 1 tablespoon of sriracha mayonnaise each.
Weigh down the sandwiches with either a heavy pan or brick wrapped in aluminum foil. Cook until golden brown, about 3 minutes. Flip the sandwiches and weight again, cooking until golden brown and the cheese is melty, about an additional 3 minutes. Remove from the pan.
Repeat with the remaining sandwiches and toppings. Open the sandwich and add a handful of arugula then close and slice in half.