Preheat oven to 375°F (190°C).
- Cook pasta until al dente according to package instructions. Drain and set aside.
- In a skillet, heat olive oil over medium heat. Sauté onion and garlic until softened, then add optional vegetables and cook until tender.
- Stir in chicken and sauce. Season with Italian seasoning, salt, pepper, and red pepper flakes if desired.
- Toss pasta with sauce mixture until evenly coated.
- Spread half the mixture in a greased 9×13-inch baking dish. Top with half the mozzarella and Parmesan.
- Add remaining pasta mixture and sprinkle with remaining cheese.
- Cover with foil and bake for 20 minutes. Remove foil and bake an additional 10–15 minutes, until golden and bubbly.
- Let rest for 5 minutes before serving. Garnish with fresh basil or parsley if desired.