Slice the mushrooms and finely chop the onion.
Melt the butter in a large pot over medium heat. Add the chopped onion and cook until softened.
Stir in the flour and cook for about 1 minute, stirring continuously.
Add the sliced mushrooms and cook until they soften and release their moisture.
Slowly pour in the hot water, stirring well to prevent lumps.
Add the heavy cream, salt, and black pepper. Stir until everything is well combined.
Let the soup simmer over low heat for about 10–15 minutes, stirring occasionally, until creamy and slightly thickened.
Serve warm.