Thinly slice the king oyster mushrooms and onion.
Melt butter in a pan and sauté the onion and mushrooms.
Dice the potato and add it to the pan; sauté together.
Add flour and cook for 1 minute.
Pour in milk and water, then bring to a boil.
Season with salt and pepper.
Once ingredients are cooked, blend smoothly with a hand blender.
Add fresh cream and reheat once more.