Preheat your oven to 400°F (200°C).
In a large skillet, cook the ground beef (or sausage) over medium heat until browned, about 5-7 minutes. Drain excess fat if necessary.
Add the diced onion and garlic powder to the skillet, cooking until the onion is translucent, about 3-4 minutes.
Stir in the pizza sauce and Italian seasoning, mixing well until heated through. Remove from heat.
In a large baking dish, spread half of the meat mixture evenly on the bottom.
Layer half of the mozzarella cheese and half of the pepperoni slices over the meat mixture.
Place frozen tater tots on top, covering the entire surface.
Spread the remaining meat mixture over the tater tots, followed by the remaining mozzarella cheese and pepperoni.
Sprinkle the grated Parmesan cheese on top.
Bake in the preheated oven for 25-30 minutes, or until the tater tots are golden brown and the cheese is bubbly.
Let the casserole cool for a few minutes before serving.