In a skillet over medium heat, add the ground beef and diced onions, reserving 1 tablespoon of raw onion for garnish.

Cook until the onions are translucent and the ground beef is browned. Drain excess grease if necessary.

Season with salt and pepper to taste.

In a bowl, mix together mayonnaise, ketchup, and mustard to create the burger sauce. Set aside.

Place about half of the tater tots into the bottom of the prepared baking dish.

Then, spread the cooked ground beef evenly over the tater tots..

Layer the diced pickles over the ground beef.

Pour the burger sauce over the pickles and spread it evenly.

Sprinkle half of the shredded cheddar cheese on top.

Arrange the frozen tater tots in a layer over the cheese.

Place the baking dish in the preheated oven and bake for about 25-30 minutes, or until the tater tots are crispy and golden brown.

Once the tater tots are cooked, remove the casserole from the oven.

Sprinkle the remaining shredded cheddar cheese over the tater tots.

Add diced tomatoes, shredded lettuce, and diced onions (if using) on top of the cheese.

Serve with additional burger sauce (homemade or store bought) if desired.