Wash mushrooms and chop finely.
Put in saucepan, add water, onion and 2 tsp salt.
Simmer 30 minutes.
Pour into sieve, saving liquid, and pressing through mushroom pulp to make a puree (or use blender if you have one).
There should be about 2 cups of the mushroom mixture.
Melt butter, blend in flour, 1 tsp salt, pepper and paprika.
Remove from heat and add milk, all at once, stirring to blend.
Return to moderate heat and cook, stirring constantly, until thick.
Add mushroom puree and heat to simmering.
Beat egg yolk and add at least half of hot mixture gradually, stirring constantly.
Stir back into pan and heat just to boiling.
Do not boil.
Serve immediately topped with whipped cream sprinkled with paprika.