Preheat the oven: Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
Cook the beef: Heat a large skillet over medium heat. Add the ground beef and diced onion. Cook for 6–8 minutes, breaking up the beef, until browned and the onion is softened. Drain excess grease.
Season the filling: Add garlic and cook for 30 seconds. Stir in pizza sauce, Italian seasoning, garlic powder, salt, and black pepper. Simmer for 2–3 minutes.
Add pepperoni and cheese: Stir in the pepperoni. Remove from heat and stir in ½ cup mozzarella and ½ cup Parmesan.
Assemble: Spread the pizza filling evenly in the prepared baking dish. Sprinkle another ½ cup mozzarella over the filling.
Prepare biscuits: Cut each refrigerated biscuit into quarters. Arrange the pieces evenly over the casserole, leaving a little space between them.
Season biscuits: Mix melted butter, Italian seasoning, garlic powder, and 2 tablespoons Parmesan. Brush over the biscuit pieces.
Bake: Bake uncovered for 25–30 minutes, until the biscuits are puffed and golden and the filling is bubbling.
Finish with cheese: Sprinkle the remaining ½ cup mozzarella over the casserole during the final 3–5 minutes of baking if you want an extra-melty cheese topping.
Rest: Remove from the oven and let rest for 5–10 minutes before serving.