Preheat Oven: Set your oven to 375°F (190°C).

Prepare Potatoes: Wash and dice potatoes into small, bite-sized pieces.

Cook Potatoes: In a skillet over medium heat, add olive oil, potatoes, and water. Season with salt and pepper. Cook until potatoes begin to soften, stirring frequently (about 5-10 minutes). Transfer to a greased baking dish.

Cook Beef Mixture: In the same pan, sauté the ground beef and diced onion until the meat is browned and the onions are soft and fragrant.

Season Mixture: Stir in the mustard along with garlic powder, onion powder, salt, and pepper to season the mixture thoroughly.

Combine in Baking Dish: Add the beef mixture to the baking dish with the potatoes. Mix well.

Add Cheese: Layer the top with shredded cheddar cheese and cover the dish with aluminum foil.

Bake: Bake in the preheated oven for 15 minutes, or until the cheese is fully melted and bubbling.

Prepare Sauce: As the casserole bakes, combine mayonnaise, ketchup, diced pickles, and pickle juice in a bowl to make the Big Mac sauce.

Add Toppings: Once baked, remove casserole from oven and foil. Drizzle Big Mac sauce over the top, then add shredded lettuce, diced tomato, chopped onion, and pickle chips.