Preheat your oven to 375°F and lightly grease a 9x13-inch baking dish.

Wash and cut potatoes into 1/2-inch cubes. Heat olive oil in a large skillet over medium heat. Add potatoes and water, season with a pinch of salt and pepper. Cook for 5-10 minutes, stirring occasionally, until potatoes are tender when pierced with a fork. Transfer to the prepared baking dish.

In the same skillet, add ground beef and chopped onions. Break up the meat with a wooden spoon and cook over medium-high heat until beef is no longer pink and onions are translucent, about 7-8 minutes. Drain excess fat if needed.

Add garlic powder, onion powder, and yellow mustard to the beef mixture. Stir well to combine and season with salt and pepper to taste. Cook for an additional 1-2 minutes to blend flavors. Add the beef mixture to the potatoes in the baking dish and mix gently.

Sprinkle shredded cheddar cheese evenly over the beef and potato mixture. Cover the dish tightly with aluminum foil and bake for 15 minutes, or until cheese is completely melted.

While the casserole bakes, prepare the Big Mac sauce. In a medium bowl, whisk together mayonnaise, ketchup, chopped pickles, and pickle juice until well combined. Refrigerate until ready to use.

Remove casserole from oven and let it rest for 5 minutes. Top with generous dollops of Big Mac sauce, then sprinkle with shredded lettuce, chopped tomatoes, diced onions, and pickle chips.

Serve immediately while hot. For meal prep, store the base casserole separately from the cold toppings and sauce. When reheating, warm only the beef and potato mixture, then add fresh toppings.