If you choose to include a sweet potato (I used a sweet potato in place of the traditional white potato because it is healthier, but for a more authentic taste and look you could use a white potato), be sure to cook it first. Add the cooked sweet potato into a food processor along with the milk, onion, mushrooms (if you want a smooth soup) and water. You can keep the mushroom slices whole of course if you prefer them that way. Process until smooth and transfer all the ingredients to a pot. Add salt and pepper to taste. Simmer on low until cooked through and serve immediately.