In a bowl mix together the milk and corn starch, whisking until incorporated. Set aside, but whisk before adding into the soup at a later time.
In a large pan on the stove on medium high pour in the olive oil, then add in the diced carrots, celery and onion and stir occasionally for 5 minutes, then add in garlic, basil, salt and thyme and stir, cooking for another 4 minutes.
In a slow cooker add in the chicken broth, milk and cornstarch mixture, chicken breast, potato gnocchi and spinach and stir to combine, and in the contents of the stovetop pan scraping down the sides of the pot.
Set the slow cooker high for 4 hours or low for 6-8 hours (all slow cookers vary a little bit, if your is hot and cooks things fast cook for 6, or if it's like mine and cooks slow cook it on low for 8 hrs).
When the timer goes off, open the lid and stir, use two forks to shred the chicken, then ladle into bowls and serve.