Preheat oven to 375°F (190°C) if baking. Bring a large pot of salted water to a boil for the pasta.

Toss chicken pieces with Italian seasoning, garlic powder, and a pinch of salt and pepper.

Boil pasta 1–2 minutes less than package directions for al dente. Reserve 1 cup pasta water, drain pasta, and set aside.

Heat 1–2 tablespoons oil in a large skillet over medium-high. Add chicken in a single layer and cook 4–5 minutes until browned and nearly cooked through.

Transfer chicken to a plate. In the same skillet, reduce heat to medium and add chopped onion. Cook 4–5 minutes until translucent, then add minced garlic and cook 30–45 seconds until fragrant.

Pour marinara into the skillet with onions. Stir and simmer 2–3 minutes. Add cooked pasta and a splash of reserved pasta water if the sauce seems thick; toss to coat.

Return chicken to the pan and fold in half the mozzarella and all the Parmesan. Taste and adjust salt and pepper.

Transfer mixture to a 9x13 baking dish, sprinkle with remaining mozzarella, and bake 15-20 minutes until cheese is bubbly and golden. For a crisp top, broil 1–2 minutes, watching closely.

Sprinkle with chopped parsley or basil and serve immediately.