Step 1: Heat olive oil in a large oven-safe skillet or Dutch oven over medium-high heat. Season chicken pieces with 1.5 teaspoons of Cajun seasoning, salt, and pepper.
Step 2: Add chicken to the hot skillet and cook until browned on all sides and cooked through, about 5-7 minutes. Remove chicken from the skillet and set aside.
Step 3: Reduce heat to medium. Add diced onion to the skillet and cook until softened, about 3-4 minutes. Add minced garlic and remaining 1/2 teaspoon Cajun seasoning, cooking for another minute until fragrant.
Step 4: Pour in chicken broth, heavy cream, and milk. Bring to a gentle simmer, scraping up any browned bits from the bottom of the skillet.
Step 5: Stir in the uncooked rotini pasta. Ensure pasta is mostly submerged in the liquid. Cover the skillet and let it simmer for 12-15 minutes, or until the pasta is al dente, stirring occasionally to prevent sticking. Add more liquid (water or milk) if the sauce becomes too thick before the pasta is cooked.
Step 6: Return the cooked chicken to the skillet. Stir in the Parmesan cheese until melted and well combined.
Step 7: Sprinkle the shredded Monterey Jack and cheddar cheese blend over the top of the pasta. Cover again for 2-3 minutes, or until the cheese is melted and bubbly.
Step 8: Garnish with fresh chopped parsley and serve hot. Taste and adjust seasoning if needed.