In a pot, add butter. Once it has melted, add onions and cook till translucent. After that, add chopped mushrooms and let it cook on medium high heat till all the moisture has evaporated.
Take some of it out for garnishing at the end.
Add salt, pepper, and all-purpose flour and mix well. Now add boiled potatoes and vegetable stock, boil it, and let it cool down.
Add the mixture to a blender and blitz it thoroughly.
Switch on the flame to low, add the blended mixture, and milk to adjust the consistency, and simmer it for 10 minutes.
Finally add the leftover cooked mushrooms, a dollop of heavy cream, and fresh basil.
Adjust the seasonings and your mushroom and potato soup is ready to be served.