I used to try doing this kinda of mushroom soup before. what i did was i buy a can of campbell's cream of mushroom first. For the smashing mushroom part. Put in some chopped shitake mushroom or button mushrooms( any mushrooms you like in fact), let it boil for like 2-3 mins then off fire.
After that pour the soup into a blender and blend it, next it back to a pot, on fire and pour in some full cream milk( you can use low fat too if you mind it's too fatty) to thicken it. Add in pinch of salt and pepper if initially you have diluted more than 1 can of water for taste.
Hope this may help you!