Season chicken breasts with onion salt and pepper, generously. Place in slow cooker.
Add wild rice, celery, carrots, onion, garlic, thyme and rosemary, and bay leaves. Season all with salt and pepper, and add Soupworks packet or onion soup mix if desired. Pour chicken stock over all of this and turn on low heat for 6-8 hours. Note: mushrooms are not added until the last hour of cooking time.
Add cremini mushrooms for the last hour of cooking time. Before adding them remove the chicken breasts and shred them with two forks. Put back into the soup. If it seems too thick, add a little more water or broth.
10-15 minutes before serving, melt butter in saucepan. Whisk in flour and stir until bubbly (about 1 minute). Whisk in milk and blend, and cook, whisking constantly, until slightly thickened (about 4 minutes). Season with salt and pepper to taste.
Add the milk mixture to crockpot and stir. Adjust thickness with blend or broth as needed.
Serve immediately, garnished with parsley. And a loaf of crusty bread with butter. Mmmmmmm!