Preheat the oven to 400°F and lightly grease a 9x13-inch baking dish to prevent sticking.
Peel and dice the onion, then cut each of the biscuits into 6 pieces.
In a large mixing bowl, stir together the melted butter, garlic, Italian seasoning, and grated parmesan cheese. Add the biscuit pieces and toss until they are evenly coated in the butter mixture.
Arrange the biscuits in a single layer in the prepared casserole dish. A few small gaps are fine, as the biscuits will expand while baking.
Pre-bake the biscuits for about 10 minutes so they start to cook and puff up slightly.
While the biscuits are baking, place the Italian sausage in a skillet over medium heat. Cook until browned, breaking it into crumbles as it cooks. Drain off any excess grease and set aside.
Remove the dish from the oven and pour the pizza sauce evenly over the partially baked biscuits. Sprinkle on half of the cooked sausage, diced onion, and pepperoni, then add half of the shredded cheese. Repeat the layers with the remaining sausage, onion, pepperoni, and cheese.
Return the casserole to the oven and bake for another 15 minutes, or until the cheese is melted and bubbly and the biscuits are fully cooked. Sprinkle a little extra Italian seasoning on top before serving.