Drizzle chicken with 1 tablespoon olive oil and season with Italian seasoning, salt, and pepper. Toss well.
Bring a large pot of salted water to a boil for the pasta.
Heat an oven-safe skillet over medium heat with olive oil. Cook chicken for 4–5 minutes per side until cooked through. Remove and set aside.
Lower heat to medium-low. Melt butter, then sauté onion until translucent. Add garlic and cook until fragrant.
Pour in milk, Cajun seasoning, red pepper flakes, salt, and pepper. Stir in sun-dried tomatoes and broccoli. Simmer until broccoli softens.
Cook rigatoni to al dente and drain.
Stir spinach into the sauce until wilted. Add pasta, chicken, and pesto, mixing until evenly coated.
Top with mozzarella and Parmesan. Broil for 3–5 minutes until melted and lightly golden.