Step 1: Brown the Beef - Set a large skillet over medium heat and add the ground beef. Cook it until it is fully browned, using a wooden spoon or sturdy spatula to break it into small crumbles as it cooks. Smaller crumbles are what you want here because they make the finished casserole easier to scoop and give every bite a little beef, cheese, and tot instead of one big chunk of meat.
Step 2: Season and Soften the Onion - Once the beef is browned, sprinkle in the salt, black pepper, garlic powder, and onion powder. Stir everything together so the seasonings coat the meat evenly. Add the diced onion and cook for about 3 to 4 minutes, just until it softens and starts smelling sweet and savory. This small step is worth taking because raw onion flavor can feel sharp, while softened onion settles right into the casserole.
Step 3: Add the Burger Flavor - Stir the yellow mustard and Worcestershire sauce into the seasoned beef. Keep stirring for a minute so the mustard coats the meat and the Worcestershire sauce has time to soak in. Drain off any extra grease before moving on. You want the beef flavorful and juicy, but you do not want a greasy casserole sitting under all that cheese.
Step 4: Prep the Slow Cooker - Lightly coat a 6 quart slow cooker with nonstick cooking spray. Do not skip this part because the cheese around the edges can stick as it melts. A quick spray makes serving easier and keeps cleanup from turning into one more thing waiting for you after dinner.
Step 5: Build the First Layers - Spread half of the seasoned beef mixture across the bottom of the slow cooker. Sprinkle half of the cheddar cheese over the beef, then add half of the torn American cheese. Arrange half of the frozen tater tots on top in an even layer. They do not need to be lined up perfectly. Just spread them out so the heat can work around them evenly.
Step 6: Repeat the Casserole Layers - Add the remaining beef over the tater tots, followed by the rest of the cheddar and American cheese. Finish with the remaining frozen tater tots. This second layer is what makes the casserole feel generous and satisfying. When you scoop down through it later, you will get those soft cheesy layers underneath and the browned tater tots waiting on top.
Step 7: Cook Until Hot and Melty - Cover the slow cooker and cook on low for 4 to 5 hours or on high for 2 to 3 hours. Try not to lift the lid too often because slow cookers lose heat quickly once opened. You are looking for melted cheese, bubbling edges, hot beef, and tater tots that are tender with some golden color where they touch the sides of the cooker.
Step 8: Add the Pickles Near the End - During the last 20 minutes of cooking, sprinkle the diced dill pickles across the top of the casserole and cover it again. Adding them late lets them warm through while keeping that briny pickle flavor noticeable. It is one of those little moves that makes a big difference when you take the first bite.
Step 9: Whisk the Creamy Sauce - While the casserole finishes, whisk the mayonnaise, French dressing, sweet pickle relish, yellow mustard, and onion powder together in a medium bowl. Keep whisking until the sauce looks smooth and creamy. Give it a taste with a clean spoon, then set it aside. It will taste even better after it sits for a few minutes and the flavors settle together.
Step 10: Rest and Finish the Casserole - Turn off the slow cooker and let the casserole rest for about 5 minutes before serving. This helps the layers settle so your scoops hold together a little better. Drizzle the top generously with Big Mac sauce, then sprinkle over the sesame seeds. Serve each portion with shredded lettuce, extra diced pickles, and another spoonful of sauce for anyone who wants it.