Arrange chicken breasts in the bottom of a large (6-quart) slow cooker. Add carrots and garlic on top, then sprinkle with parsley, basil, celery salt, nutmeg, and bay leaf. Season with ¼ to ½ teaspoon salt (depending on the saltiness of your chicken broth) and freshly ground black pepper. Pour chicken stock/broth over ingredients in slow cooker, cover with lid, and cook on LOW for 6 to 8 hours or until the chicken is tender.
At the end of the cooking time, remove the chicken to a cutting board and discard the bay leaf. Using a fork, whisk the cornstarch into the half-and-half until dissolved and then stir the slurry into the soup, along with the gnocchi. Cover the slow cooker and increase the heat to HIGH.
Chop or pull the chicken into bite-sized pieces. Stir it back into the slow cooker, cover, and continue to cook for 15 to 30 more minutes, until the gnocchi is cooked through and the soup is thickened. Just before serving, stir in the chopped spinach and cover for a few more minutes until wilted. Adjust the seasonings, to taste, with additional salt and pepper (if needed) and serve immediately.
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Notes
For a lighter soup, use fat-free half-and-half or whole milk. On the other hand, you may use heavy cream to make it more decadent.
You may use frozen gnocchi instead of fresh or packaged. Just be prepared to slightly increase the cooking time at the end until it's cooked through.
You may add an additional tablespoon of cornstarch if you prefer an even thicker soup. Do not stir cornstarch directly into the hot soup or it will clump up. Instead, whisk cornstarch into a small amount of cool liquid (water, broth, milk) until dissolved. Then gradually stir the resulting slurry into your hot soup before allowing it time to thicken.