**1. Make the Spread:** Mix mayonnaise, relish, ketchup, sugar, and vinegar in a small bowl until it turns pink. Set aside.
**2. Prepare Toppings:** Slice raw onions, tomatoes, and pepperoncini peppers. Tear iceberg lettuce leaves.
**3. Optional – Caramelize Onions:** Finely dice 1 large onion. In a pan over low-medium heat, add vegetable oil, onions, and salt. Stir continuously for about 12 minutes until golden brown. Increase heat to medium, add water, and repeat until onions are dark golden brown and translucent. Set aside.
**4. Prepare the Patties:** Divide ground beef into 8 portions (3 to 4 oz each). Gently flatten each portion into desired thickness, seasoning with salt and pepper. Set aside.
**5. Toast the Buns:** Melt 2 tablespoons of butter. Brush butter onto bun halves and toast in a pan until golden brown.
**6. Cook the Patties:** In a skillet over medium-high heat, add butter and vegetable oil. Cook patties for 2-3 minutes until browned, flipping once. Add mustard and cheese, cooking for another minute.
**7. Assemble the Burgers:** Spread the prepared mixture onto the bottom bun. Layer pickles, tomatoes, lettuce, patty with cheese, onions, and peppers. Spread more mixture onto the top bun and seal the burger.
**8. Enjoy your homemade In-N-Out experience!**