Preheat the oven to 375°F (190°C). Wash and chop the potatoes into small bite-sized pieces, leaving the skin on.

In a pan over medium heat, add olive oil and the chopped potatoes. Pour in 1/4 cup of water. Season generously with salt and pepper. Cook until the potatoes start to soften, stirring frequently for about 5-10 minutes.

Transfer the softened potatoes to a baking dish.

In the same pan, cook the ground beef and onions until the beef is mostly cooked through and the onions are softened. Season with salt, pepper, garlic powder, onion powder, and mustard. Mix well. Add the cooked beef mixture to the potatoes in the baking dish and combine.

Sprinkle shredded cheddar cheese over the potato and beef mixture. Cover the dish with foil and bake for 15 minutes until the cheese melts.

While the casserole bakes, prepare the Big Mac sauce by mixing mayonnaise, ketchup, chopped pickles, and pickle juice. Refrigerate until ready to use.

Once the casserole is done baking, remove it from the oven and the foil. Drizzle the Big Mac sauce over the top, adding shredded lettuce, chopped white onion, chopped tomatoes, and pickle chips for garnish.

Serve and enjoy your delicious Big Mac Casserole!