Slice the onions and chop the mushrooms into small pieces.

Melt the butter in a large pot, then add the onions and mushrooms. Cook over low heat for about 15 minutes.

Add the flour and stir well. Pour in the water and crumble in half a bouillon cube. Add the milk and simmer for about 15 minutes, stirring occasionally.

Blend the soup with an immersion blender. When serving, add a little lemon juice, some chopped green scallion, and a dollop of heavy cream.