Heat the oven: Preheat to 375°F (190°C). Grease a 9x13-inch baking dish.
Par-cook the pasta: Boil in salted water for 2 minutes less than package directions.
Drain and set aside.
Season the chicken: Pat dry and toss with 1/2 teaspoon salt, black pepper, and half the Italian seasoning.
Sear the chicken: In a large skillet, heat 1 tablespoon oil over medium-high. Brown chicken 2–3 minutes per side until just cooked through. Transfer to a bowl.
Sauté aromatics: Add remaining oil.
Cook onion 3–4 minutes until translucent. Stir in garlic, sun-dried tomatoes, remaining Italian seasoning, and red pepper flakes for 30 seconds.
Make the sauce: Pour in chicken broth and bring to a simmer. Reduce heat, stir in cream and Parmesan until smooth.
Taste and adjust salt and pepper.
Add greens: Stir in spinach until just wilted, 30–60 seconds.
Combine: In the baking dish, mix pasta, chicken, and sauce. Top with mozzarella and a little extra Parmesan.
Bake: Cover loosely with foil and bake 15 minutes. Uncover and bake 8–10 more minutes, until bubbly and lightly golden.
Rest and serve: Let sit 5 minutes.
Garnish with chopped basil or parsley. Serve warm.