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.
Finely mince onion, celery, carrots, garlic and 8 ounces of the mushrooms in a food processor or by hand and transfer to slow cooker crock.
Thinly slice remaining mushrooms and add to slow cooker pot.
Puree cannellini beans with the liquid from the can 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 varying 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 a a few tablespoons of the soup to the half and half and stir to temper. Add to the crock pot with the 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.