1. Season chicken breasts with salt and pepper and place them in the slow cooker.
2. Add the chopped onion, minced garlic, Italian seasoning, chicken broth, cream of mushroom soup, and white wine to the slow cooker.
3. Cover and cook on low for 4 hours.
4. Remove the chicken from the slow cooker and shred it with two forks. Return the shredded chicken to the slow cooker.
5. Stir in the heavy cream, spaghetti noodles, grated Parmesan cheese, and sour cream. Make sure the pasta is submerged in the liquid.
6. Cover and continue cooking for another 30 minutes to an hour, or until the pasta is tender.
7. About 10 minutes before serving, stir in the thawed peas and sliced mushrooms.
8. Let everything heat through, then serve with a sprinkle of extra Parmesan if desired.
