Step 1: Add the diced onion, chopped carrots, chopped celery, boneless skinless chicken breasts, garlic powder, minced fresh garlic, dried thyme, dried rosemary, and chicken broth to your slow cooker.
Step 2: Cover and cook on low for 6-8 hours, or on high for 3-4 hours, until the chicken is cooked through and tender.
Step 3: Carefully remove the cooked chicken breasts from the slow cooker and shred them using two forks. Return the shredded chicken to the slow cooker.
Step 4: In a small bowl, whisk together the evaporated milk and cornstarch until smooth to create a slurry. Pour this mixture into the slow cooker and stir well to combine.
Step 5: Add the potato gnocchi and baby spinach to the slow cooker. Stir gently to ensure the gnocchi is submerged and the spinach starts to wilt.
Step 6: Continue to cook on high for an additional 30-45 minutes, or until the gnocchi are tender and the soup has thickened to your desired consistency. Stir occasionally to prevent gnocchi from sticking.
Step 7: Taste the soup and season with salt and freshly ground black pepper as needed before serving hot.