01 - Preheat oven to 400°F. Grease a 9×13-inch baking dish or line with parchment paper to prevent sticking.
02 - In a large skillet over medium heat, cook ground beef with diced onion, breaking up the meat until browned and onion is soft, about 5-7 minutes. Drain excess grease with a slotted spoon, leaving only the cooked beef and onion.
03 - Return beef mixture to the skillet if removed. Add marinara sauce, Italian seasoning, salt, and black pepper. Stir to combine and simmer for 2 minutes, allowing flavors to meld.
04 - Spread the beef and sauce mixture evenly in the prepared baking dish. Sprinkle 1 cup shredded mozzarella cheese over the mixture.
05 - Arrange frozen tater tots in a single layer over the cheese and beef mixture. Top the tater tots with remaining 1 cup shredded mozzarella cheese.
06 - Bake in the preheated oven for 25-30 minutes until tater tots are golden brown and cheese is melted and bubbly.
07 - If desired, scatter mini pepperoni, olives, bell pepper, green onions, or mushrooms over the top. Bake for an additional 5 minutes until toppings are warmed through.
08 - Allow casserole to cool slightly before serving. Portion into squares and serve while hot.