Preheat the oven to 400°F (200°C) and lightly grease a 9×13-inch baking dish.

Heat a skillet over medium. Add the ground beef and diced onion. Cook, breaking the beef into pieces, for 5 to 7 minutes until browned. Drain excess grease.

Return the skillet to medium heat. Stir in the marinara sauce, Italian seasoning, and a pinch of salt and pepper. Let the mixture simmer for a couple of minutes to meld flavors.

Spread the beef and sauce evenly in the prepared baking dish. Smooth it into an even layer.

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

Arrange the frozen tater tots in a single layer on top of the cheese. Fit them snugly so the top bakes evenly.

Sprinkle the remaining 1 cup of mozzarella over the tater tots.

Bake for 25 to 30 minutes, until the tater tots are golden and the cheese is melted and bubbly.

If you want toppings, scatter mini pepperoni, olives, bell pepper, green onions, or mushrooms over the casserole and return it to the oven for 4 to 5 minutes to warm them through.

Let the casserole rest for 5 minutes before serving so it sets slightly and slices hold together.