Rinse wild rice under cold water until the water runs clear. Drain and transfer to a 5- or 6-quart slow cooker crock.
To a food processor, add onion, celery, carrots and garlic. Pulse everything to a very fine mince and add to slow cooker crock.
Add 8 oz. mushrooms to the food processor and pulse until they’re finely chopped. Thinly slice the additional 4 oz. mushrooms. Add all mushrooms to the slow cooker.
Puree beans until smooth and pour into slow cooker with other ingredients. Add bay leaf, garlic powder, and 6 cups of the vegetable stock. Season with salt and pepper to taste. Stir to combine all ingredients.
Cover and cook on low for 6-8 hours or high for 3-4 hours. Begin checking for doneness after 5 ½ hours on low or 2 ½ hours on high (cooking time will vary by slow cooker). Adjust cook time accordingly. The rice will be tender but not mushy when done.
When the soup is fully cooked, remove and discard bay leaf. Add tempered half-and-half, parsley, and remaining vegetable stock (if necessary) to achieve your ideal consistency. Stir to combine all ingredients.
Add additional salt and pepper to taste. Serve immediately.
Use a multi-color wild rice blend.Rinse and drain the beans to reduce the recipe's sodium content.Temper the half-and-half before adding it to the slow cooker.