In a large skillet over medium heat, brown the ground beef until no longer pink, about 5-7 minutes. Add in the diced onion and minced garlic, sautéing until the onion is translucent. Stir in the Worcestershire sauce along with salt and pepper to taste. Let it simmer on low for another 2 minutes, then remove it from heat and set aside.

Preheat your oven to 350°F (175°C). Tear the hamburger buns into smaller pieces and place them in a large bowl. Melt the butter and drizzle it over the bun pieces, tossing to coat evenly. Sprinkle with sesame seeds and garlic powder. Spread the buns on a baking sheet and toast them in the oven for 5 minutes until golden brown.

In a medium bowl, combine the mayonnaise, ketchup, and sweet pickle relish. Add the yellow mustard, white vinegar, sugar, and paprika. Whisk until the sauce is smooth, then taste and adjust the ingredients to your liking, setting it aside for layering.

Grease a 9x13-inch baking dish with cooking spray or butter. Start by layering half of the cooked beef mixture in the dish. Next, add slices of American cheese, followed by pickles and diced onions. Spoon about ⅓ of the special sauce over the top, then scatter the toasted bun pieces and drizzle with remaining sauce. Finish with the rest of the beef and top with shredded cheddar cheese.

Place the assembled Big Mac casserole in the preheated oven and bake uncovered for 20-25 minutes, or until the cheese is melted, bubbly, and beginning to brown around the edges.

Remove the casserole from the oven and allow it to cool for about 5 minutes to help it set. Top with shredded lettuce, extra pickles, and onions if desired. Drizzle with any remaining special sauce and serve warm.