Season chicken with salt and pepper, to taste. Place chicken into a 6-qt slow cooker.
Stir in chicken stock, wild rice, garlic, onion, carrots, celery, thyme, rosemary and bay leaves; season with salt and pepper, to taste. Cover and cook on low heat for 6-8 hours. Add mushrooms during the last 30 minutes of cooking time.
Remove chicken from the slow cooker and shred, using two forks.
Melt butter in a saucepan over medium heat. Whisk in flour until lightly browned, about 1 minute. Whisk in milk and half and half, and cook, whisking constantly, until slightly thickened, about 4-5 minutes; season with salt and pepper, to taste.
Stir chicken and milk mixture into the slow cooker. If the soup is too thick, add more half and half as needed until desired consistency is reached.
Serve immediately, garnished with parsley, if desired.
Notes
*Half and half is equal parts of whole milk and cream. For 1 cup half and half, you can substitute 3/4 cup whole milk + 1/4 cup heavy cream or 2/3 cup skim or low-fat milk + 1/3 cup heavy cream.