Preheat your oven to 400°F (200°C) and grease a 9x13-inch baking dish with cooking spray or olive oil.

In a large skillet, sauté ground beef and diced onion over medium heat for 5-7 minutes until meat is browned and onion is translucent.

Drain any excess grease, return mixture to heat, stir in marinara sauce, Italian seasoning, salt, and pepper; simmer for 2-3 minutes.

Spread the beef mixture evenly across the prepared baking dish and sprinkle 1 cup of shredded mozzarella cheese over the top.

Arrange frozen tater tots in a single layer on top of the cheese, then sprinkle the remaining mozzarella cheese over the tater tots.

Bake for 25-30 minutes until tater tots are golden brown and the cheese is bubbling.

Let the casserole cool for 5-10 minutes before slicing and serving.