Combine vegetable stock, wild rice, mushrooms, garlic, carrots, celery, sweet potato, onion, bay leaf and Old Bay seasoning in the bowl of a large slow cooker. Stir briefly to combine, then place the lid on the slow cooker.
Cook on high for 2-3 hours*, until the rice is cooked and tender.
Meanwhile, during those final 10 minutes of slow cooking, prepare your cream sauce on the stove. In a medium saucepan, cook the butter over medium-high heat until melted. Whisk in the flour until combined and cook for 1 minute. Gradually add in the milk and whisk until combined. Continue cooking, stirring frequently, until the mixture nearly comes to a simmer and has thickened. (It should be very thick.)
Add the cream sauce and kale to the soup and stir gently until combined. Taste and season with salt and pepper (plus any extra Old Bay seasoning, if you would like) as needed.
Serve warm. Or transfer to sealed container(s) and refrigerate for up to 4 days.