Preheat oven to 350 degrees F. Spray a 9x13-inch baking dish with nonstick cooking spray; set aside.

Cook pasta according to package directions; drain and set aside.

Heat olive oil in a large skillet over medium-high heat. Add turkey and onion to the pan and cook, breaking up meat, until turkey is cooked through and onion is tender. Add garlic and cook for an additional minute. Stir in marinara and mix until well combined then remove from heat.

Reserve a handful of pepperoni (approx. 20) to place on top of the casserole before baking and dice the rest. Add diced pepperoni, cooked pasta and half the mozzarella cheese to the skillet and stir until well combined. Spread pasta mixture in prepared pan and top with remaining mozzarella, parmesan cheese and whole pepperoni.

Cover with foil and bake for 30 minutes. Remove foil and bake an additional 5 minutes, or until cheese is melted and golden brown. Garnish with fresh parsley before serving, if desired.