In a medium sized frying pan, melt 1 1/2 ounces [3 tablespoons] of the butter over moderate heat.
When the foam subsides, add the mushrooms and mix well.
Pour in the water and add 1 teaspoon of the salt, the lemon juice, nutmeg and black pepper.
Bring the mixture to the boil, stirring frequently.
Simmer, stirring occasionally, for 8 minutes.
Remove the pan from the heat and strain the mushrooms and cooking liquid into a large mixing bowl.
Reserve the cooking liquid and set aside.
Puree the mushrooms in a blender or food mill, or, alternatively, mash them with a fork.
Set aside.
Add the remaining butter to the frying pan and melt it over moderate heat.
Remove the pan from the heat and, with a wooden spoon, stir in the flour to make a smooth paste.
Gradually add the reserved cooking liquid and the milk, stirring constantly.
Return the pan to the heat.
Simmer the mixture, stirring constantly, for 2 minutes or until it is thick.
Stir the pureed mushrooms into the sauce and cook, stirring, for 2 minutes.
Add the remaining salt if required.
Remove the pan from the heat and transfer the puree to a warmed serving dish.
Sprinkle over the parsley and arrange the croutons decoratively around the puree.