Prepare mussels and inspect them carefully.
Place in a pan with onion, sprig parsley, water (slightly less water if adding white wine) and seasoning.
Heat gently until mussels open.
Take off...
onions and peel about half the mushrooms, leaving the stalks on.
Fry these mushrooms and the onions in butter, and place on the bottom of the flan case.
Put grated cheese in a bowl, if used, season...
