Preheat and prep the dish. Heat the oven to 375°F (190°C). Lightly oil a 9x13-inch baking dish or coat with cooking spray.

Cook the pasta. Boil in salted water until just shy of al dente (1–2 minutes less than package time). Drain and set aside. The pasta will finish cooking in the oven.

Sauté the aromatics. In a large skillet, warm olive oil over medium heat. Add onion and bell pepper; cook 4–5 minutes until softened. Stir in mushrooms and cook 2–3 minutes more. Add garlic and cook 30 seconds until fragrant.

Brown the turkey. Push the veggies to the side. Add ground turkey, season with salt, pepper, and Italian seasoning. Cook, breaking it up, until browned and no longer pink, about 6–8 minutes.

Add the sauce. Pour in marinara or pizza sauce. Add red pepper flakes if using. Simmer 3–4 minutes so flavors meld. Taste and adjust seasoning.

Combine with pasta. In a large bowl, mix the cooked pasta with the turkey-sauce mixture. Fold in half the mozzarella, half the Parmesan, and most of the olives and pepperoni if using. Save a little for the top.

Assemble the casserole. Spread the mixture into the baking dish. Top with remaining mozzarella and Parmesan. Scatter any reserved pepperoni and olives on top.

Bake. Bake uncovered for 18–22 minutes, until the cheese is melted and the edges are bubbling. For extra browning, broil 1–2 minutes at the end, watching closely.

Rest and garnish. Let it stand 5–10 minutes so it sets. Sprinkle with chopped basil or parsley before serving.