Grilled Mussels Tapas “Rockefeller”

Mussels

medium onion

Water for boiling & salt

For the bechamel
milk
Cooking water of mussels
All purpose flour
A smidge of nutmeg
White pepper

To coat with breadcrumbs

beaten egg yolks
breadcrumbs

Parmesan cheese

Chives 
olive oil

Make sure bechamel has the consistency of heavy cream. Grill on high until golden brown.