Cook On Low. Cover and cook on LOW for 3 to 4 hours, checking smaller or thinner breasts earlier. Cooking time depends on the thickness of the chicken and your slow cooker. Check the thickest part with an instant-read thermometer. The chicken is done when it reaches 165°F. Use temperature rather than the clock alone to decide when it is ready.

Cook The Pasta Separately. Near the end of the chicken's cooking time, cook the fettuccine or penne according to the package directions. Before draining, scoop out 1 cup of pasta water and set it aside. Drain the pasta when it reaches your preferred texture.

Slice Or Shred The Chicken. Transfer the cooked chicken to a cutting board. Slice it into strips or shred it with two forks, then cover it to keep warm. Sliced chicken gives you distinct pieces in each serving. Shredded chicken mixes throughout the pasta and sauce.

Finish The Homemade Alfredo Sauce. Stir the heavy cream into the cooking liquid. Cover and heat on HIGH for about 15 to 20 minutes, until hot.

Switch the slow cooker to WARM. Gradually stir in the freshly grated Parmesan until melted. Add it in small handfuls rather than dumping it in all at once.

Combine And Serve. Return the chicken to the slow cooker and add the drained pasta. Toss gently until everything is coated.

Let stand for 5 minutes so the sauce can thicken slightly. If it becomes too thick, stir in reserved pasta water a little at a time. Taste and adjust the seasoning, then garnish with parsley.