In a large skillet, heat a tablespoon of oil over medium heat. Sauté one pound of ground beef until it's half-cooked, approximately 5 minutes, breaking it up as it cooks. Add diced onions and minced garlic, continuing to cook until the beef is fully browned and the onions are translucent, about an additional 5 minutes. Drain excess fat, then stir in Worcestershire sauce, yellow mustard, salt, pepper, garlic powder, and onion powder. Let it simmer for 5 minutes.

Preheat your oven to 350°F (175°C). While it’s warming, tear four hamburger buns into bite-sized pieces in a large bowl. Melt a couple of tablespoons of butter and drizzle it over the bun pieces, tossing them with sesame seeds and garlic powder until they are evenly coated. Bake the bun pieces on a baking sheet for about 5 minutes, or until they’re lightly golden and crispy.

In a mixing bowl, combine the mayonnaise, ketchup, sweet pickle relish, white vinegar, sugar, paprika, garlic powder, and onion powder. Stir until all ingredients are well blended, adjusting flavors to your preference.

Grease a 9x13-inch baking dish with cooking spray or oil. Layer half of the cooked beef mixture at the bottom, followed by a layer of American cheese, pickles, and half of the special sauce. Next, add half of the toasted bun pieces on top. Repeat with the remaining beef, cheese, toasted buns, and drizzle the last of the special sauce over the top for extra flavor.

Place the assembled casserole in the preheated oven and bake uncovered for about 20 minutes or until the cheese is bubbly and the edges appear crispy and golden.

Once the Big Mac Casserole is done baking, remove it from the oven and let it cool slightly for 5-10 minutes. Top each serving with fresh shredded lettuce and any remaining pickles or special sauce.