Rinse the wild rice under cold running water until water runs clear.
Let drain and transfer to a 5 or 6-quart slow cooker crock.
Add onion, celery, carrots, and garlic to a food processor and pulse until very finely minced and transfer to slow cooker crock.
Add 8 oz. mushrooms to food processor and pulse until finely chopped.
Thinly slice remaining mushrooms and add to slow cooker pot, along with the finely chopped mushrooms.
Puree cannellini beans until smooth and pour into slow cooker crock with other ingredients. Add bay leaf, garlic powder, and 6 cups of vegetable stock.
Season with salt and black pepper, to taste, and stir to thoroughly combine all ingredients.
Cover and cook on high for 3-4 hours or low for 6-8.
Actual cooking time will vary by individual slow cooker, so check for doneness after 2½ hours (or 5½ hours for low) and adjust cooking time accordingly.
(The rice should be tender, but not mushy when finished).
When cooking time is complete, remove bay leaf and discard.
Add tempered half and half, chopped parsley, and additional vegetable stock, if necessary, to achieve the desired consistency.
Stir to combine thoroughly.
Season with additional salt and black pepper, to taste, and serve immediately.
Enjoy!