Add the chicken breast, chopped onion, garlic parmesan sauce, cream cheese, milk, and parmesan cheese to your slow cooker.
Set the slow cooker to low and cook for 4-6 hours, until the chicken is fully cooked and tender.
About 30 minutes before serving, cook the rotini pasta according to the package instructions. Drain and set aside.
Shred or chop the cooked chicken directly in the slow cooker. Add the cooked rotini pasta to the slow cooker, mixing well to coat the pasta in the creamy sauce.
Let everything cook for an additional 15 minutes on low to allow the flavors to combine.