Place the diced onions in the bottom of a slow cooker.
Lay the chicken breasts on top of the onions.
Add the chopped spinach and chopped artichoke hearts over the chicken.
Add the cream cheese, chopped garlic, onion powder, salt, pepper, and chicken broth to the slow cooker.
Cover with the lid and cook on HIGH for 4 hours.
After 3.5 hours, carefully remove the chicken breasts from the slow cooker.
Shred the chicken using two forks, then return the shredded chicken to the pot and stir well.
Stir in the half and half, mozzarella, and parmesan cheese.
Replace the lid and continue cooking for about 30 minutes.
Remove the lid and give the soup one final good stir.
Serve and enjoy!