Place the chicken breasts in the slow cooker along with the onion, carrots, celery, garlic, chicken broth, basil, Italian seasoning, thyme, poultry seasoning, salt, and black pepper.
Cover and cook on LOW for 6 to 7 hours or until the chicken is tender and cooked through. You can also cook it on HIGH for about 3½ to 4 hours.
Transfer the chicken to a plate and shred it with two forks. Return the shredded chicken to the slow cooker.
Whisk the cornstarch and cold water together in a small bowl until completely smooth. Stir the mixture into the soup along with the evaporated milk.
Add the gnocchi and stir gently to combine. Cover and cook on HIGH for about 30 to 45 minutes, or until the gnocchi are tender and the soup has thickened.
While the gnocchi cook, heat a skillet over medium heat. Add the chopped bacon and cook until crisp. Transfer to a paper towel-lined plate, leaving about 1 teaspoon of bacon fat in the skillet.
Add the minced garlic to the skillet and cook for about 30 seconds. Add the spinach and sauté until just wilted.
Stir the spinach and crispy bacon into the slow cooker. Taste and adjust the seasoning with additional salt and black pepper if needed.
Ladle the hot soup into bowls and finish with freshly grated Parmesan and chopped parsley. Serve with crusty bread for dipping.