Preheat oven to 400℉.In a large skillet, prepare diced hash browns according to package directions. Remove from pan and set aside.
In the same pan that you cooked the hash browns, heat oil over medium heat. Add onions and ground beef and cook until beef is cooked through and onions are soft. Season with Worcestershire sauce, salt, and pepper.
Add the diced hash browns back to the pan with the beef and stir until combined. Remove from heat and add 1 cup of shredded cheddar cheese. Stir until cheese is melted.
Spray a 9x13 baking dish with cooking spray. Spread beef/hash brown mixture evenly over the bottom of the pan. Sprinkle remaining cheddar cheese over the top. Cover with foil and bake for 30 minutes.
While casserole is baking, prepare the Big Mac sauce by stirring all the sauce ingredients together until smooth and well combined. Set aside.
Remove casserole from oven and allow to cool, uncovered for 3-5 minutes.
Top with lettuce, pickles, tomatoes, white onion, and Big Mac Sauce. Serve immediately.
