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 patty(s), pressing 4 ounces of beef flat to 4-inches in diameter for a single patty. Sprinkle both sides with salt and freshly ground pepper.
Heat griddle or heavy skillet over medium-high heat. Add 2 teaspoons olive oil. Sear patties on one side. Squirt some mustard on them as they cook. After about 2 minutes, flip the patties over so the mustard cooks into the second side. Cover each beef patty with a slice of cheese. Cook until internal temperature reaches 145°F on an instant-read thermometer. Remove to a plate and cover loosely with foil.
While patties are frying, place closed bun in the hot oven for exactly 2 minutes.
To caramelize the onion (if using), chop finely and then allow to cook in the meat juices that have formed. Turn onion often so it becomes brown and caramelized, but not burned.
Open the bun and place it on griddle or skillet to "toast" in some of the meat juices. Watch carefully to avoid burning.
Spread 2 tablespoons of the prepared spread on the bottom, toasted bun. Top with four dill pickle slices, followed by a slice of tomato and lettuce. Lay one beef patty on top, followed by the onion and the second beef patty. Close with the top bun.
Enjoy your Double-Double Cheeseburger, In-N-Out style! For a Single Cheeseburger, use only one beef patty and one slice of cheese.