Preheat oven to 375F and lightly spray a 9x13 casserole dish. Set aside.
Make your Big Mac sauce by combined all the ingredients into a small bowl and whisk well.
Add ground meat to a large pan over medium heat until almost fully cooked.
Strain out some excess fat, then add onion, garlic powder, salt and pepper and cook for another 5 minutes.
Transfer beef to prepared casserole dish, spread about ½ the sauce (or more) on top of the meat mixture, then top with half of the cheese.
Place an even layer of frozen tater tots over top of the cheese, and bake uncovered, for 35-40 minutes or until the tater tots are golden and crispy.
Top with the remaining cheese and bake for another 3-5 or until melted.
Cool slightly, then top with lettuce, pickles, Big Mac sauce, and sesame seeds.