Begin to preparing the diced onion and banana peppers, all toppings for burger assembly, and the In-N-Out spread.
Toast the buns well in a large skillet or on a griddle. Optional: spread 1/2-1 tablespoon of light mayo on the buns for extra browning. Set the toasted buns aside.
Divide the ground beef into four 2-ounce portions. Don't over handle or roll into balls like meatballs. Add a pinch of salt and black pepper and set aside.
Cooking the Burgers
Heat your skillet or griddle over medium-high heat.
Once hot, carefully place the ground beef, one portion at a time, in the skillet. Place a small square of parchment paper or a large non-stick spatula on top of the beef and press firmly into the skillet. You want to smash them into a thin, lacy-edged patty. Repeat with the remaining beef.
Add the diced onion, chopped banana peppers, and yellow mustard on top of two of the four patties.
Continue cooking until a dark brown crust forms on the bottom of each patty before flipping and adding a slice of American cheese to each patty. (Don't worry about losing some onion/chiles when flipping.)
Cook for another minute or so after flipping. Once the cheese has melted, place the patty with the onion and peppers on top of the other patty and remove from the heat.
Add spread on both buns, place the lettuce, tomato, and pickles on the bottom bun, followed by the burgers. Close, serve, and enjoy!