Preheat oven to 400°F.
Arrange and assemble all of the ingredients.
Make the spread: mix spread ingredients together in a small bowl. Set aside.
Form beef patties, pressing 4 ounces of beef flat to 4 inches in diameter for each patty. Sprinkle both sides with salt and pepper.
Heat griddle or heavy skillet over medium-high heat. Add 2 teaspoons olive oil. Sear patties on one side. Squirt mustard on them as they cook. After about 2 minutes, flip so the mustard cooks into the second side. Cover each patty with a slice of cheese. Cook until internal temperature reaches 145°F. Remove to a plate and cover loosely with foil.
While patties are frying, place the closed bun in the hot oven for exactly 2 minutes.
To caramelize the onion, if using, chop finely and cook it in the meat juices left in the pan, stirring often so it browns without burning.
Open the bun and toast it, cut-side down, in the remaining meat juices. Watch it closely.
Spread 2 tablespoons of sauce on the toasted bottom bun. Top with pickle chips, tomato, and lettuce. Add one patty, then the onion, then the second patty. Close with the top bun.
Enjoy your Double-Double, In-N-Out style. For a Single, use one patty and one slice of cheese.