Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish with olive oil or cooking spray.
In a large skillet, heat the olive oil over medium heat. Add the frozen diced hash brown potatoes and cook according to the package instructions until they are golden brown and crispy. Spread the cooked hash browns evenly into the prepared baking dish.
In the same skillet, add the ground beef. Season the beef mixture with garlic powder, salt, and pepper to taste. Cook over medium heat until the beef is browned add the onions and cook until they are soft and translucent. Drain any excess fat.
Spread the cooked ground beef mixture over the layer of hash browns in the baking dish.
Next, sprinkle the grated cheddar cheese over the top of the casserole.
Place in the preheated oven and bake for 20-25 minutes, or until the cheese is melted.
While baking the casserole, prepare the Big Mac sauce by mixing the mayonnaise, ketchup, chopped dill pickles, and dill pickle juice in a small bowl. Set aside.
Once the casserole is cooked and the cheese is melted, remove it from the oven and let it cool slightly.
Sprinkle the shredded lettuce over the top of the cooked casserole, followed by chopped tomatoes, finely chopped onions, and dill pickle slices.
Drizzle the prepared Big Mac sauce evenly over the top of the garnished casserole and sprinkle with sesame seeds and more diced onions if desired..
Serve hot and enjoy your delicious Big Mac Casserole!