Preheat the oven to 400 degrees. Grease a 9x13 inch casserole dish.
Peel and dice the onion.
Cut each of the biscuits into 6 smaller pieces.
In a large bowl, combine the melted butter, garlic, seasoning, and parmesan. Drop the cut up biscuits into the bowl and toss well to coat all of the biscuit pieces.
Dump the biscuits into the greased casserole dish and arrange in an even layer. Remember they will puff as they bake, so it doesn’t matter if there are small gaps between the biscuits.
Transfer the casserole dish to the oven and bake for 10 minutes.
While the biscuits are pre-baking, cook the italian sausage in a skillet over medium heat, breaking it apart as it cooks. Drain any grease and set aside.
Take the biscuits out of the oven, and pour the sauce over the top, spreading all the way across the casserole dish. Top with half of the italian sausage, onion and pepperoni, then sprinkle half the cheese. Repeat those layers.
Return the casserole dish to the oven and bake for an additional 15 minutes, and sprinkle a little extra Italian seasoning over the top, then serve.