Prep the crockpot: Lightly grease the insert with butter or olive oil. This helps the sauce stay silky and prevents sticking.
Season the chicken: Pat the chicken dry.
Sprinkle with salt, pepper, Italian seasoning, paprika, and onion powder on both sides.
Layer the base: Add the minced garlic and onion to the bottom of the slow cooker. Place the seasoned chicken on top.
Add liquids: Pour in the chicken broth. If using diced tomatoes, add them now.
Dot the top with the cream cheese (cut into chunks).
Cook low and slow: Cover and cook on Low for 3.5–4.5 hours or High for 2–3 hours, until the chicken is cooked through and easily shreds.
Shred the chicken: Remove the chicken to a plate and shred with two forks. Return it to the slow cooker and stir to melt and blend the cream cheese.
Make it creamy: Stir in the heavy cream (or half-and-half) and the shredded parmesan. Taste and adjust salt and pepper.
Cook the pasta: Boil the pasta in salted water according to package directions until al dente.
Drain well. (Avoid cooking dry pasta in the slow cooker; it tends to over-soften.)
Combine: Add the drained pasta to the slow cooker and toss to coat in the sauce. If using spinach or kale, fold it in now so it wilts gently.
Finish and serve: Let everything sit on Warm for 5–10 minutes to marry the flavors. Top with chopped parsley or basil and extra parmesan.
Serve hot.