Preheat the oven to 375°F. Wash and chop unpeeled potatoes into small, bite-sized pieces.
Heat olive oil in a pan over medium heat. Add the potatoes and 1/4 cup of water. Season generously with salt and pepper and cook for 5 to 10 minutes, stirring frequently to prevent burning, until softened. Transfer the potatoes to a baking dish.
Add ground beef and onions to the same pan. Cook until the beef is mostly browned and onions are soft. Season with salt, pepper, garlic powder, onion powder, and mustard. Stir well, then combine the beef mixture with the potatoes in the baking dish.
Top the potato and beef mixture with shredded cheddar cheese and cover the dish with foil. Bake for 15 minutes until the cheese has melted.
Prepare the Big Mac sauce by whisking together mayonnaise, ketchup, chopped pickles, and pickle juice. Refrigerate the sauce until the casserole is ready.
Remove the casserole from the oven and discard the foil. Drizzle the entire portion of sauce over the top and garnish with shredded lettuce, chopped white onion, chopped tomatoes, and pickle chips. Serve immediately.
