Preheat your oven to 425°F and generously grease a 9×13-inch baking dish.

Sauté the ground beef and diced onion over medium heat in a large skillet until the beef is browned and the onion is tender, about 5-7 minutes. Drain any excess grease.

Combine the pizza sauce, Italian seasoning, garlic powder, salt, and pepper into the skillet with the beef mixture. Stir thoroughly and let it simmer for a couple of minutes.

Spread the beef and sauce mixture evenly in the prepared baking dish.

Sprinkle 1 cup of the shredded mozzarella cheese generously over the beef layer.

Arrange the frozen tater tots in a single layer over the cheese and beef mixture.

Top with the remaining 1 cup of mozzarella cheese and optional pepperoni slices.

Bake the casserole in the preheated oven for 30 minutes, until the tater tots are golden brown and crispy.

Cool slightly after removing from the oven, then serve.